Docs: Add reST directives to c-api/intro#Include_Files by aeros · Pull Request #14698 · python/cpython

Adds the following reST directives to the "Include Files" section: code c, important, and note. Also improves the phrasing in the section where a note was added, on lines 99-101.

Since this change is quite minor and doesn't affect the actual message conveyed, I decided to go without creating an issue for it. It's mostly formatting related and an improvement to the phrasing (splits a run-on sentence into two individual ones). This could also go without a news entry, so the skip news and skip issue labels would probably be appropriate.