Add "API" to all respective headings (h1s)

doc/api/award_emoji.md

-# Award Emoji
+# Award Emoji API
 > [Introduced][ce-4575] in GitLab 8.9, Snippet support in 8.12

doc/api/boards.md

-# Boards
+# Issue Boards API
 Every API call to boards must be authenticated.

doc/api/branches.md

-# Branches
+# Branches API
 ## List repository branches

doc/api/broadcast_messages.md

-# Broadcast Messages
+# Broadcast Messages API
 > **Note:** This feature was introduced in GitLab 8.12.

doc/api/build_variables.md

-# Build Variables
+# Build Variables  API
 ## List project variables

doc/api/ci/lint.md

-# Validate the .gitlab-ci.yml
+# Validate the .gitlab-ci.yml (API)
 > [Introduced][ce-5953] in GitLab 8.12.

doc/api/ci/runners.md

-# Runners API
+# Register and Delete Runners API
 API used by Runners to register and delete themselves.

doc/api/deploy_key_multiple_projects.md

-# Adding deploy keys to multiple projects
+# Adding deploy keys to multiple projects via API
 If you want to easily add the same deploy key to multiple projects in the same
 group, this can be achieved quite easily with the API.

doc/api/deploy_keys.md

-# Deploy Keys
+# Deploy Keys API
 ## List all deploy keys

doc/api/milestones.md

-# Milestones
+# Milestones API
 ## List project milestones

doc/api/namespaces.md

-# Namespaces
+# Namespaces API
 Usernames and groupnames fall under a special category called namespaces.

doc/api/notes.md

-# Notes
+# Notes API
 Notes are comments on snippets, issues or merge requests.

doc/api/pipeline_triggers.md

-# Pipeline triggers
+# Pipeline triggers API
 You can read more about [triggering pipelines through the API](../ci/triggers/README.md).

doc/api/projects.md

 # Projects API
-
 ### Project visibility level
 Project in GitLab has be either private, internal or public.
@@ -17,8 +16,6 @@ Constants for project visibility levels are next:
 * `public`:
   The project can be cloned without any authentication.
-
-
 ## List projects
 Get a list of visible projects for authenticated user. When being accessed without authentication, all public projects are returned.

doc/api/repositories.md

-# Repositories
+# Repositories API
 ## List repository tree

doc/api/repository_files.md

-# Repository files
+# Repository files API
 **CRUD for repository files**

doc/api/services.md

-# Services
+# Services API
 ## Asana

doc/api/session.md

-# Session
+# Session API
 ## Deprecation Notice

doc/api/sidekiq_metrics.md

-# Sidekiq Metrics
+# Sidekiq Metrics API
 >**Note:** This endpoint is only available on GitLab 8.9 and above.

doc/api/system_hooks.md

-# System hooks
+# System hooks API
 All methods require administrator authorization.