Improve docs formatting and update links.

[bdice]

Description

This PR improves a variety of small issues in the docs, including misaligned headers that cause poor rendering, outdated links, and inconsistent use of code formatting. I am hoping to make a few more passes through the docs to make them more useful and move from the deprecated recommonmark to MyST as was recently done in cuDF. This PR is a starting point that is probably worth merging on its own.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

@gpucibot merge