[docs] document Prestissimo reduce_agg behavior

[steveburnett]

Description

Adds documentation of the behavior of reduce_agg in Prestissimo as requested in Issue 21179.

To contain this and future related content, this PR adds a new page "Prestissimo Limitations" to the Prestissimo Developer Guide documentation - parallel to the existing page "Prestissimo Features".
(The Prestissimo Developer Guide documentation was restructured in PR 21953.)

My thanks to @tdcmeehan for providing the draft explanation of the behavior of reduce_agg in Prestissimo.

Motivation and Context

Fixes Issue 21179.

Impact

Documentation.

Test Plan

Screenshots of the local build of documentation:

1. Prestissimo Developer Guide index page, showing new Limitations page link in left navbar and also in the page (center, bottom).

2. Limitations page, showing the reduce_agg description in context.

Contributor checklist

• Please make sure your submission complies with our development, formatting, commit message, and attribution guidelines.
• PR description addresses the issue accurately and concisely. If the change is non-trivial, a GitHub Issue is referenced.
• Documented new properties (with its default value), SQL syntax, functions, or other functionality.
• If release notes are required, they follow the release notes guidelines.
• Adequate tests were added if applicable.
• CI passed.

Release Notes

== NO RELEASE NOTE ==

[tdcmeehan]

Can this use a relative URL?

[steveburnett]

Good catch! Yes it can.

Edited RST file locally to use relative URL (../functions/aggregate.html#reduce_agg), made a new local build, and tested link with relative URL.

• The link displays correctly on the page.
• Selecting the link takes me to the correct destination.

Updated PR and squashed commits.