doc/user: use Materialize-dialect SQL syntax highlighting

[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. The most recent Hugo release includes it! So, this PR:

1. Upgrades Hugo to the latest stable version. I had to make a slight update to our footer to account for .File being nil on pages without a directly attributable file (e.g., directories without an _index.md).
2. Applies 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! If you'd prefer I introduce this section by section I can break up the PR.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• 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:

[arusahni]

This has seemingly revealed a change in how Hugo computes relative URLs for links when the baseURL is a subdirectory. There's quite a robust discussion in the Hugo community around this. It seems as if the guidance is to convert relative Markdown URLs to use cross references. So,

[secrets](/sql/create-secret/)

becomes

[secrets]({{< ref "/sql/create-secret/" >}})

When I patch our render-link hook to add the /docs prefix, the linter is far happier. I wonder if we'd be best suited by punching through some custom config that the hook reads and applies to the URLs (or extracts the subdir from the baseURL), and then gradually shifting to the (more correct) cross reference impl.

[arusahni]

Closing. Landed the Hugo changes in #27679 , and will get the syntax highlighting changes staged in a separate PR.