Add MkDocs configuration to enable GitLab Pages documentation build

Problem/Motivation

The docs/ directory and all documentation pages exist in the repository, but there is no mkdocs.yml file. Without it, the DrupalCI pages job included via
include.drupalci.main.yml cannot build or publish the documentation.

Proposed resolution

Add a mkdocs.yml at the project root using the MkDocs Material theme, configured with the site name, repo URL, and a nav structure that reflects the existing docs/ directory hierarchy.

• Add mkdocs.yml with site name, repo URL, Material theme, and nav structure reflecting the existing docs/ hierarchy

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
• ✅ Testing to ensure no regression
• ✅ Automated unit testing coverage
• ✅ Automated functional testing coverage
• ➖ UX/UI designer responsibilities
• ✅ Readability
• ✅ Accessibility
• ✅ Performance
• ✅ Security
• ✅ Documentation
• ✅ Code review by maintainers
• ✅ Full testing and approval
• ✅ Credit contributors
• ✅ Review with the product owner
• ✅ Release notes snippet
• ✅ Release canvas_override-1.0.0-alpha2

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• feat: #3583263 Add MkDocs configuration to enable GitLab Pages documentation build