Previously, when a namespace was configured, built-in PHP types like
`null` were incorrectly prefixed with the namespace (e.g.,
`ActivityPub\null`). This fix:

- Adds `maybe_prefix_namespace()` helper to centralize namespace logic
- Excludes all built-in types: null, mixed, void, false, true, callable,
  resource, iterable
- Adds test case to verify null and other built-in types are not prefixed

Dynamic hooks (e.g., 'prefix_' . $var) are now properly documented:
- Hook name/slug uses {$var} syntax (e.g., activitypub_inbox_{$type})
- Example code uses wildcard * (e.g., activitypub_inbox_*)

Implementation:
- Detects concatenation operator (.) after hook string
- Extracts variable names and appends as {$var}
- Added extract_dynamic_parts() helper to extract concatenated variables
- Added get_hook_name_for_example() helper to replace {$var} with *
- Works with both 'prefixed' and 'default' example styles

Tests:
- Added test_dynamic_hook_extraction() to verify hook name format
- Added test_dynamic_hook_documentation_uses_wildcard() to verify example format
- All 39 tests passing

This commit adds comprehensive support for detecting and documenting deprecated WordPress hooks that use do_action_deprecated() and apply_filters_deprecated().

Features:
- Detects deprecated hooks with version and replacement information
- Generates deprecation warnings in documentation with version info and replacement guidance
- Shows deprecated hooks with strikethrough formatting in index
- Handles all deprecation scenarios: with/without version, with/without replacement
- Comprehensive test coverage with expected output verification