[docs] Add RoaringBitmap SQL functions to aggregation 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?