HBASE-29878 Improve the Documentation UI

[yuriipalam]

As discussed earlier here https://lists.apache.org/thread/0ojmobpnyk44lbghqxh2c231c3bm6t6t
I’m presenting the new documentation website. You can check this out here:
https://hbase-landing.vercel.app/docs/

What’s been done:

• Converted all asciidoc files to mdx (extended markdown). 99% of the content preserved, checked it manually multiple times.
• Multi-page by default, single-page as an alternative option for those who got used to it.
• Multi-page has a powerful search. The whole website is static and pre-rendered at a build time.
• “Edit on GitHub” button is at the bottom of each section that leads directly to the source of the section. Doesn’t work in the demo website because it leads to the original repo, and the change is not merged yet.
• Dark mode support.
• The UI now looks modern and beautiful, the user/dev experience is improved significantly.
• The landing page and documentation are now one application.
• The PDF generation is integrate into the build process. PDFs are also available in both Dark and Light modes.
• The old docs are still available at "/book.html" with a banner that the page is legacy, so people with bookmarks/notes won't get the 404 error.

Thanks!