COMDOX-345: Add topic metadata and validation test

Purpose of this pull request

This pull request (PR):

Adds keyword metadata to enhance the site search experience for users (fcd6a66331769d6e58242b8b20c5a3469ef9f604).
- These keywords will appear as additional filters on the site search results page so that users can narrow the scope of what they're looking for.
- Keywords are defined in the .github/linters/metadata.schema.yml file.
- Keyword values map to the exl-config/features.yml metadata file that we use for Experience League repos for consistency.
Adds a new PR validation test for the following metadata (93e6cb403abd9498c6c90191b46f23f14e69f6a9):
- title: is required and cannot exceed 60 characters (SEO)
- description: is required and cannot exceed 160 characters (SEO)
- keywords: is optional and must match a defined value (site search filtering)

See COMDOX-345 for details.