.gitlab/CODEOWNERS additions

Description

1. Following !1950 (merged), add exclude-modules.txt and exclude-scripts.txt to .gitlab/CODEOWNERS as these are used by test_python_imports.py and test_python_scripts.py
2. Add *.md files under build/packaging/support files (requiring maintainer approval). This should mostly capture README.md files in various locations which describe parts of the build system, which maintainers should be able to update easily without needing package-level approvals. Maintainer approval for package-specific *.md files should also help keep a consistent format/style across e.g. README.md files.

API Changes and Justification

Backwards Compatible Changes

• This change does not modify any class/function/struct/type definitions in a public C header file or any Python class/function definitions
• This change adds new classes/functions/structs/types to a public C header file or Python module

Backwards Incompatible Changes

• This change modifies an existing class/function/struct/type definition in a public C header file or Python module
• This change removes an existing class/function/struct/type from a public C header file or Python module

Review Status

cc @adam-mercer @duncanmmacleod