HBASE-29878 Improve the Documentation UI (addendum)

[PDavid]

• Ignore .mdx files in RAT check. The problem is that every MDX file starts with the frontmatter section and the docs library parses it expecting it to be at the start.
• Fix licenses of old book.html and it's CSS files.
• Ignore the generated hbase-website/app/pages/_docs/docs/_mdx/(multi-page)/configuration/hbase-default.md file from spotless check
• Install playwright OS package dependencies in Yetus General Check GitHub Actions workflow based on this: https://playwright.dev/docs/ci-intro#setting-up-github-actions

[PDavid]

The problem is that every MDX file starts with the frontmatter section:

---
title: "Preface"
description: "This is the official reference guide for the HBase version it ships with."
---

The docs library parses it expecting it to be at the start.

[yuriipalam]

Many thanks! 👍