docs: clarify built-in and custom rules

[kvlonge]

Why

The Rules overview repeated its opening review examples as a second abstract question list. It also did not clearly explain how built-ins and custom Rules serve different adoption needs, and the SQL responsibility table could imply that SQBRSQL runs outside compilation.

Changes

• replace the repetitive question list with a concise built-in/custom distinction
• describe built-ins as individually selectable opinionated defaults and examples
• explain that custom Rules capture project- or company-specific decisions too narrow for universal defaults
• clarify that SQBRSQL Rules are executed by the compiler
• clarify that sqb format remains separate from the compiler

Verification

• public-repository hygiene scan
• git diff --check
• manual review of the rendered MDX structure and links