Split mccabe --max-complexity into its own extension section

[SemTiOne]

Type of Changes

	Type
✓	📜 Docs

Description

Two checkers share name = "design", so --max-complexity merged into the standard Design section. The collision is static (one extension reuses a core name), so we hardcode frozenset({"design"}) instead of spawning linter instances to detect it. An optional options_anchor kwarg on BaseChecker.get_full_documentation() lets the doc generator pass the correct anchor; callers that don't pass it keep the old behavior.

Closes #9174

[SemTiOne]

When I look at it again, I shouldn't hardcode frozenset({"design"}). I'll replace it with collision detection computed from the registered linter (_colliding_checker_names) so that any future core/extension name clash is handled automatically.