Moves files in `templates/zerver/help/include` that are used
specifically for API documentation pages to be in a new directory:
`templates/zerver/api/include`.
Adds a boolean parameter to `render_markdown_path` to be used
for help center documentation articles.
Also moves the test file `empty.md` to the new directory since
this is the default directory for these special include macros
that are used in documentation pages.