fix: Put the main content of the pages in a main landmark

Problem/Motivation

The pages of the Website Starter site template have no main landmark. The page layout prints the main content in a plain UIkit section, so assistive technologies cannot jump to the main content of /blog, the blog posts, the contact form and the other pages.

Steps to reproduce

1. Install Drupal with the Website Starter site template.
2. Open /blog and look for a <main> element or role="main".

Proposed resolution

• Give the main content section of the default page layout the role="main" attribute.

Remaining tasks

• ✅ File an issue about this project
• ❌ Addition/Change/Update/Fix to this project
• ❌ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ UX/UI designer responsibilities
• ❌ Accessibility and Readability
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors

User interface changes

None visible. The main content of every page is a main landmark.

API changes

None.

Data model changes

None.