[docs] Add RoaringBitmap SQL functions documentation

[pbanakar]

Purpose

Linked issue: Part of #3289

Adds a dedicated SQL Functions reference page for the 12 RoaringBitmap SQL functions introduced by FIP-37 (PRs #3319, #3398, #3492, #3777). These are Flink-side functions registered by FlussCatalog and are distinct from the storage-level rbm32/rbm64 aggregators.
Documenting them on the Aggregation Merge Engine page blurred the ownership boundary, so this PR moves the content to a dedicated page.

Brief change log

Added website/docs/engine-flink/sql-functions.md:

• New page titled "SQL Functions" placed after Delta Joins in the Flink engine sidebar
• Opens with a "How to Use" section explaining catalog setup and how to enable the functions
• Documents all 12 functions (4 aggregate, 8 scalar) with signature, null semantics table,
  and a self-contained deterministic VALUES-based example showing expected output
• Notes where functions have no server-side counterpart (rb_and_agg, rb_xor_agg)

Modified website/docs/table-design/merge-engines/aggregation.md:

• Removed the RoaringBitmap SQL Functions section
• Added a short cross-reference tip block next to rbm32/rbm64 pointing to the new page

Tests

Documentation-only change. Verified locally by running the website dev server.
SQL examples verified against implemented functions in PRs #3319, #3398, #3492, and #3777.

API and Format

No API or storage format changes. Documentation only.

Documentation

Yes — this PR adds the user-facing function reference for FIP-37, completing the documentation commitment made in PRs #3319, #3398, #3492, and #3777.

[pbanakar]

@polyzos @platinumhamburg Could you please take a look at this PR?

[wuchong]

Thanks @pbanakar , I left some comments.

[wuchong]

rb_build does not return NULL when all array elements are null. The implementation returns a serialized empty bitmap for any non-null array, and the existing test explicitly verifies that an all-null array produces a non-null empty bitmap. Therefore, rb_build(ARRAY[CAST(NULL AS INT)]) IS NULL is actually false, and rb_cardinality(...) returns 0. Please update the documentation to state that only a null array returns NULL; empty arrays and all-null arrays return an empty bitmap.

[pbanakar]

Fixed rb_build null semantics: a NULL array returns NULL; an empty array or all-null array returns an empty bitmap

[pbanakar]

Thanks @wuchong for review, this looks more structured and well organized than before, PTAL another look when you have some time thanks!

[wuchong]

The null-handling tables take up a large portion of the page and feel repetitive, while the same rules would be easier to scan as short prose. Please use the earlier compact format and add a **Null Handling:** ... item alongside **Signature:** for each function instead of using a table.

[wuchong]

Please build these inputs with rb_build rather than hard-coded serialized bytes, for example by placing arrays in VALUES and applying rb_build before rb_or_agg. This keeps the example self-explanatory and avoids opaque binary literals whose contents readers cannot verify or safely modify.

[pbanakar]

Hi @wuchong addressed all comments, ready for another look, Thanks 🙏

[wuchong]

Thanks @pbanakar , I added 2 commits to improve the SQL examples. LGTM.