Docs: Add reST directives to c-api/intro#Include_Files

[aeros]

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.

[aeros]

Thanks for the review @benjaminp.

I've removed the code directive, removed the note directive you recommended removing, and replaced the previous important directive I was using with note, which seems to be more appropriate based on the devguide's recommended usage of it. Let me know what you think of the changes.

When I had initially opened this PR, I was significantly less familiar with the usage of reST within the docs. I've gained some experience since then, and likely would not have opened a PR applying those initial changes.

[miss-islington]

Thanks @aeros167 for the PR, and @benjaminp for merging it 🌮🎉.. I'm working now to backport this PR to: 3.7, 3.8.
🐍🍒⛏🤖

[bedevere-bot]

GH-15872 is a backport of this pull request to the 3.8 branch.

[bedevere-bot]

GH-15873 is a backport of this pull request to the 3.7 branch.

[aeros]

Thanks @benjaminp!