Add "API" to API docs
What does this MR do?
Add the word "API" to APIs docs' h1 headings to improve docs.gitlab.com site search results
This is a test. If successful, we'll add it to all h1 headings
Related: https://gitlab.com/gitlab-com/gitlab-docs/issues/102, https://gitlab.com/gitlab-com/gitlab-docs/issues/77