api: UI improvements

[jorgeorpinel]

Emanates from #908

• Support ```py Markdown highlighter (code blocks)

• Automatic link to function names in back quotes (similar to command names) to their api ref. Eg. `dvc.api.read()` -> /doc/api-reference/read

• (maybe) special CSS for first <h3> (###) in api ref pages, currently used as title No longer needed, see api: create docs #908 (comment).

• Better way to show what literals in example code blocks evaluate to. Currently print() is used in the block, then a --- (<hr>) and a following block quote (```dvc) shows what the output of running the script would do:
  

[shcheklein]

Closing in favor of #910 ... everything else is solved more or less I believe