doc/user: convert all user docs to use mzsql highlighting where appropriate

[arusahni]

Motivation

• This PR adds a feature that has not yet been specified.

Syntax highlighting in our docs uses a general SQL highlighter. This is fine, but we don't have highlighting on Mz-specific keywords, which is suboptimal. I have upstreamed Materialize SQL dialect highlighting to Chroma, the highlighting library used by Hugo. We upgraded to a compatible version of Hugo in #27679 , so now we just need to apply the new syntax highlighting to all examples of Materialize SQL dialect in the docs. I took care to not update SQL Server, MySQL, and Postgres examples.

Before

After

If we're happy with this, and once it's merged, I'll work on formalizing the Chroma dialect update motion as a script so we can upstream new keywords as they land.

Tips for reviewer

Sorry for the large diff! I grouped sections of the docs by commit, so that may ease review. Please let me know if you'd prefer I submit this another way.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes:

[morsapaes]

Thank you so much for this, @arusahni! ❤️ Also for correctly highlighting the PostgreSQL- and MySQL-specific bits.