Add docs for code intelligence

Document code navigation features

doc/user/project/code_intelligence.md

+---
+type: reference
+---
+
+# Code Intelligence
+
+> [Introduced](https://gitlab.com/groups/gitlab-org/-/epics/1576) in GitLab 13.1.
+
+Code Intelligence adds code navigation features common to interactive
+development environments (IDE), including:
+
+- Type signatures and symbol documentation.
+- Go-to definition
+
+Code Intelligence is built into GitLab and powered by [LSIF](https://lsif.dev/)
+(Language Server Index Format), a file format for precomputed code
+intelligence data.
+
+## Configuration
+
+Enable code intelligence for a project by adding a GitLab CI/CD job to the project's
+`.gitlab-ci.yml` which will generate the LSIF artifact:
+
+```yaml
+code_navigation:
+  script:
+    - go get github.com/sourcegraph/lsif-go/cmd/lsif-go
+    - lsif-go
+artifacts:
+  reports:
+    lsif: dump.lsif
+```
+
+The generated LSIF file must be less than 170MiB.
+
+After the job succeeds, code intelligence data can be viewed while browsing the code:
+
+![Code intelligence](img/code_intelligence_v13_1.png)
+
+## Language support
+
+Generating an LSIF file requires a language server indexer implementation for the
+relevant language.
+
+| Language | Implementation |
+|---|---|
+| Go | [sourcegraph/lsif-go](https://github.com/sourcegraph/lsif-go) |
+| JavaScript | [sourcegraph/lsif-node](https://github.com/sourcegraph/lsif-node) |
+| TypeScript | [sourcegraph/lsif-node](https://github.com/sourcegraph/lsif-node) |
+
+View a complete list of [available LSIF indexers](https://lsif.dev/#implementations-server) on their website and
+refer to their documentation to see how to generate an LSIF file for your specific language.

doc/user/project/index.md

@@ -104,6 +104,7 @@ When you create a project in GitLab, you'll have access to a large number of
 - [Dependency List](../application_security/dependency_list/index.md): view project dependencies. **(ULTIMATE)**
 - [Requirements](requirements/index.md): Requirements allow you to create criteria to check your products against. **(ULTIMATE)**
 - [Static Site Editor](static_site_editor/index.md): quickly edit content on static websites without prior knowledge of the codebase or Git commands.
+- [Code Intelligence](code_intelligence.md): code navigation features.
 
 ### Project integrations