Cody completions: Add docs page (#52447)

Shipping a simple docs page about completions with some guide on how to
enable them.

If this needs to include some marketing specific language, please let me
know 😅

## Test plan

Docs page, view rendered here:
https://github.com/sourcegraph/sourcegraph/pull/52447/files?short_path=32b8c0b#diff-32b8c0b3a5643a0a3039005e355f016e2f3436afb77f4b062eb48c91721c00d5

<img width="1534" alt="Screenshot 2023-05-25 at 13 55 16"
src="https://github.com/sourcegraph/sourcegraph/assets/458591/91ab793f-2bc6-4eaf-b1db-a0846bc2b129">



<!-- All pull requests REQUIRE a test plan:
https://docs.sourcegraph.com/dev/background-information/testing_principles
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---------

Co-authored-by: Erik Seliger <erikseliger@me.com>
Co-authored-by: Alex Isken <alex.isken@sourcegraph.com>
Co-authored-by: Thorsten Ball <mrnugget@gmail.com>

.metals/metals.log

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+2023.05.26 14:31:06 INFO  Started: Metals version 0.11.12 in workspace '/Users/philipp/dev/sourcegraph' for client Visual Studio Code 1.78.2.
+May 26, 2023 2:31:06 PM org.flywaydb.core.internal.license.VersionPrinter printVersionOnly
+INFO: Flyway Community Edition 9.16.3 by Redgate
+May 26, 2023 2:31:06 PM org.flywaydb.core.internal.license.VersionPrinter printVersion
+INFO: See release notes here: https://rd.gt/416ObMi
+May 26, 2023 2:31:06 PM org.flywaydb.core.internal.license.VersionPrinter printVersion
+INFO: 
+May 26, 2023 2:31:06 PM org.flywaydb.core.internal.database.base.BaseDatabaseType createDatabase
+INFO: Database: jdbc:h2:file:/Users/philipp/dev/sourcegraph/.metals/metals (H2 2.1)
+May 26, 2023 2:31:06 PM org.flywaydb.core.internal.schemahistory.JdbcTableSchemaHistory allAppliedMigrations
+INFO: Schema history table "PUBLIC"."flyway_schema_history" does not exist yet
+May 26, 2023 2:31:06 PM org.flywaydb.core.internal.command.DbValidate validate
+INFO: Successfully validated 4 migrations (execution time 00:00.006s)
+May 26, 2023 2:31:06 PM org.flywaydb.core.internal.schemahistory.JdbcTableSchemaHistory create
+INFO: Creating Schema History table "PUBLIC"."flyway_schema_history" ...
+May 26, 2023 2:31:06 PM org.flywaydb.core.internal.command.DbMigrate migrateGroup
+INFO: Current version of schema "PUBLIC": << Empty Schema >>
+May 26, 2023 2:31:06 PM org.flywaydb.core.internal.command.DbMigrate doMigrateGroup
+INFO: Migrating schema "PUBLIC" to version "1 - Create tables"
+May 26, 2023 2:31:06 PM org.flywaydb.core.internal.command.DbMigrate doMigrateGroup
+INFO: Migrating schema "PUBLIC" to version "2 - Server discovery"
+May 26, 2023 2:31:06 PM org.flywaydb.core.internal.command.DbMigrate doMigrateGroup
+INFO: Migrating schema "PUBLIC" to version "3 - Jar symbols"
+May 26, 2023 2:31:06 PM org.flywaydb.core.internal.command.DbMigrate doMigrateGroup
+INFO: Migrating schema "PUBLIC" to version "4 - Fingerprints"
+May 26, 2023 2:31:06 PM org.flywaydb.core.internal.command.DbMigrate logSummary
+INFO: Successfully applied 4 migrations to schema "PUBLIC", now at version v4 (execution time 00:00.024s)
+2023.05.26 14:31:06 INFO  time: initialize in 0.3s
+2023.05.26 14:31:07 WARN  Build server is not auto-connectable.
+2023.05.26 14:31:07 WARN  the build tool Bazel is not supported by Metals, please open an issue if you would like to contribute to improve the situation.
+2023.05.26 14:47:27 INFO  Shutting down server
+2023.05.26 14:47:27 INFO  shutting down Metals
+2023.05.26 14:47:27 INFO  Exiting server

doc/cody/completions.md

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+# Cody completions
+
+<aside class="experimental">
+<p>
+<span class="badge badge-experimental">Experimental</span> This feature is experimental and might change or be removed in the future. We've released it as an experimental feature to provide a preview of functionality we're working on.
+</p>
+</aside>
+
+![Example of Cody completions. You see a code snippet starting with async function getWeather(city: string) { and Cody response with a multi-line suggestion using a public weather API to return the current weather ](https://storage.googleapis.com/sourcegraph-assets/cody_completions.png)
+
+## What are Cody completions?
+
+Cody provides real-time code completions as you're typing. As you start coding, or after you type a comment, Cody will look at the context around your open files and file history to predict what you're trying to implement and provide completions. It's autocomplete powered by Cody!
+
+## Enabling Cody completions
+
+While in experimental state, Cody completions need to be enabled manually. To do that:
+
+1. Make sure your [Cody AI by Sourcegraph](https://marketplace.visualstudio.com/items?itemName=sourcegraph.cody-ai) extension is on the latest version
+   - <kbd>shift</kbd>+<kbd>cmd</kbd>+<kbd>x</kbd> to see all extensions, select Cody, confirm the version
+1. Go to the Cody Extension Settings and enable completions
+   - Click to check the box for: `Cody > Experimental Suggestions`
+1. Finally, restart or reload VS Code and test it out!
+
+### Configuring on Sourcegraph Enterprise
+
+Please follow the steps in [Enabling Cody on Sourcegraph Enterprise](explanations/enabling_cody_enterprise.md#enabling-cody-on-sourcegraph-enterprise) to enable Cody on Sourcegraph Enterprise.
+
+You have to configure the model used for Cody completions via the `completionModel` option inside the `completions` site config.
+
+```json
+{
+  // [...]
+  "completions": {
+    "enabled": true,
+    "provider": "anthropic",
+    "completionModel": "claude-instant-v1.0",
+    "accessToken": "<key>"
+  }
+}
+```
+
+> NOTE: Self-hosted customers need to update to a minimum of version 5.0.4 to use completions.
+
+> NOTE: Cody completions currently only work with Claude Instant or our LLMProxy configured with Claude Instant. Support for other models will be coming later.

doc/sidebar.md

@@ -75,6 +75,7 @@ Keep it as a single list with at most 2 levels. (Anything else may not render co
   - [Quickstart](cody/quickstart.md)
   - [Explanations](cody/explanations/index.md)
   - [FAQ](cody/faq.md)
+  - [Completions (experimental)](cody/completions.md)
 - [App (beta)](app/index.md)
 - [Own (experimental)](own/index.md)
 - <br/>