Add titles to every readme page + add test to validate it

[zastrowm]

Description

I'm incrementally working towards getting our docs working with astro.build - to do so markdown files need a title front-matter.

Currently, mkdocs does this to derive the title:

The "title" to use for the document.

MkDocs will attempt to determine the title of a document in the following ways, in order:

1. A title defined in the nav configuration setting for a document.
2. A title defined in the title meta-data key of a document.
3. A level 1 Markdown header on the first line of the document body.
   (Setext-style headers are supported only since MkDocs 1.5.)
4. The filename of a document.

Upon finding a title for a page, MkDoc does not continue checking any additional sources in the above list.

However, astro.build desires the title to be in the front-matter; for compatibility with both, add the title from the nav and add it as a frontmatter meta property. Also added a test to enforce this for now

Type of Change

• New content addition

Motivation and Context

Incrementally let's docs work for both astro.build and mkdocs

Checklist

• I have read the CONTRIBUTING document
• My changes follow the project's documentation style
• I have tested the documentation locally using mkdocs serve
• Links in the documentation are valid and working
• Images/diagrams are properly sized and formatted
• All new and existing tests pass

Additional Notes

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.