Fix broken links in documentation

Also, move the old linting docs to the new location.

doc/api/README.md

@@ -55,15 +55,10 @@ following locations:
 - [Tags](tags.md)
 - [Todos](todos.md)
 - [Users](users.md)
-- [Validate CI configuration](ci/lint.md)
+- [Validate CI configuration](lint.md)
 - [V3 to V4](v3_to_v4.md)
 - [Version](version.md)
 
-The following documentation is for the [internal CI API](ci/README.md):
-
-- [Builds](ci/builds.md)
-- [Runners](ci/runners.md)
-
 ## Road to GraphQL
 
 Going forward, we will start on moving to

doc/api/lint.md

+# Validate the .gitlab-ci.yml (API)
+
+> [Introduced][ce-5953] in GitLab 8.12.
+
+Checks if your .gitlab-ci.yml file is valid.
+
+```
+POST /lint
+```
+
+| Attribute  | Type    | Required | Description |
+| ---------- | ------- | -------- | -------- |
+| `content`  | string    | yes      | the .gitlab-ci.yaml content|
+
+```bash
+curl --header "Content-Type: application/json" https://gitlab.example.com/api/v4/ci/lint --data '{"content": "{ \"image\": \"ruby:2.1\", \"services\": [\"postgres\"], \"before_script\": [\"gem install bundler\", \"bundle install\", \"bundle exec rake db:create\"], \"variables\": {\"DB_NAME\": \"postgres\"}, \"types\": [\"test\", \"deploy\", \"notify\"], \"rspec\": { \"script\": \"rake spec\", \"tags\": [\"ruby\", \"postgres\"], \"only\": [\"branches\"]}}"}'
+```
+
+Be sure to copy paste the exact contents of `.gitlab-ci.yml` as YAML is very picky about indentation and spaces.
+
+Example responses:
+
+* Valid content:
+
+    ```json
+    {
+      "status": "valid",
+      "errors": []
+    }
+    ```
+
+* Invalid content:
+
+    ```json
+    {
+      "status": "invalid",
+      "errors": [
+        "variables config should be a hash of key value pairs"
+      ]
+    }
+    ```
+
+* Without the content attribute:
+
+    ```json
+    {
+      "error": "content is missing"
+    }
+    ```
+
+[ce-5953]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5953
+