[docs] Add RoaringBitmap SQL functions documentation

[pbanakar]

Purpose

Linked issue: Part of #3289

Adds user-facing documentation for the 12 RoaringBitmap SQL functions introduced by FIP-37 (PRs #3319, #3398, #3492, #3777). The functions are now registered natively in FlussCatalog and available after USE CATALOG fluss_catalog without any CREATE TEMPORARY FUNCTION statement.

Brief change log

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

Added a new ## RoaringBitmap SQL Functions section after rbm64, covering all 12 functions across two subsections:

Aggregate functions: rb_build_agg, rb_or_agg, rb_and_agg, rb_xor_agg

Scalar functions: rb_cardinality, rb_build, rb_contains, rb_to_array, rb_or, rb_and, rb_xor, rb_andnot

Each function entry includes signature, behavior, null semantics, and a Flink SQL example. Functions without a server-side counterpart (rb_and_agg, rb_xor_agg) include a note about compaction behavior. A note distinguishes these Flink-side SQL functions from the storage-level rbm32/rbm64 aggregators already documented in the file.

Closes with an end-to-end example covering table creation, ingestion with rb_build_agg, point query with rb_cardinality, and roll-up query with rb_or_agg, with a reference to the Real-Time UV Deduplication blog post for a full Docker-based tutorial.

Tests

Manually verified by starting the website locally , The SQL examples have been verified against the implemented functions in PRs #3319, #3398, #3492, and #3777.

API and Format

This change does not affect any API or storage format. Documentation only.

Documentation

Yes, this PR adds the user-facing reference documentation for FIP-37 bitmap SQL functions, 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]

These are Flink-side SQL functions registered by the Fluss catalog, not storage-level aggregate functions executed by the Fluss merge engine, so documenting them in the Aggregation Merge Engine page blurs the ownership and execution boundary. Could we move this section to a dedicated website/docs/engine-flink/sql-functions.md page titled SQL Functions, place it after Delta Joins in the Engine Flink sidebar, and leave only a short cross-link next to rbm32/rbm64 here?

For each function, please make the example self-contained and show the expected output. Prefer deterministic VALUES inputs so readers can immediately connect the inputs, output, and documented behavior. Also, please rename End-to-End Example to How to Use and move to the beginning of the page, and use that section to explain how to enable and use these Fluss-provided Flink functions, followed by the broader usage example.

[pbanakar]

• Moved the entire section to a new dedicated page website/docs/engine-flink/sql-functions.md, placed after Delta Joins in the Flink sidebar
• Added a short cross-link in aggregation.md next to rbm32/rbm64 pointing to the new page
• Rewrote "End-to-End Example" as "How to Use" and moved it to the top of the page to explain catalog setup first
• All examples now use deterministic VALUES inputs and show explicit expected outputs

[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!