feat: Improve cast compatibility tests and docs

[andygrove]

Which issue does this PR close?

Part of https://github.com/apache/datafusion-comet/issues/286

Rationale for this change

What changes are included in this PR?

• Improved CometCastSuite to assert that cast tests are enabled (and therefore passing) for casts that we report as compatible. This helps ensure that the docs are in sync with the real status. There is likely more that we can do in the future but this is another step closer.
• Update compatibility reporting based on errors found by this new test enhancement
• Added some notes on some of the current incompatibilities so that they are in the documentation
• Improvements to the documentation formatting
• Fixed a regression from a recent PR where the cast float/double to string tests weren't actually running the test

Docs Preview

We have a table of compatible casts, followed by:

How are these changes tested?

Tested docs locally.

[andygrove]

@viirya @snmvaughan This PR incorporates some of the feedback from https://github.com/apache/datafusion-comet/pull/362