[core] Support FM indexes for primary-key tables

[JingsongLi]

Purpose

Support exact FM contains indexes for primary-key tables and document FM index usage.

Changes

• add PK FM configuration, validation, schema guards, and query routing
• build FM payloads in source-row order and append bounded partitions to one checksummed container file
• keep one payload file and one IndexFileMeta per index group while preserving partition-level demand reads
• add exact Contains coverage for partial index states, short needles, Unicode, updates, and deletes
• add FM documentation under docs/docs and the documentation sidebar

Tests

• 24 focused FM format and query tests
• 132 focused paimon-core tests
• 4 Spark 3 PrimaryKeySortedIndexTest cases
• paimon-core non-fast compile/checkstyle/spotless/enforcer
• docs sidebar syntax check