Initialize Webtheme Default Content module

Problem/Motivation

Needed to default content for menu links and custom block content.

Proposed resolution

Add default content for menu links and custom block content module.

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
• ✅ Merge request, Patch, or Commit
• ✅ 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 webtheme_default_content-10.0.0

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Issue #3341506 by Tasneem Natshah: Initialized Webtheme Default Content module