Docs various improvements

[maximlt]

Fixes #433
Fixes #379

• The logo links to holoviz.org
• Removed right side bar on the landing page
• Categories that have no project (e.g. Neurosciences at the moment) aren't created
• Migrate doc files from rst to Markdown (there's still rst to build the grids as its nested structure help there)
• Re-add project-level toctree when a project has multiple files
• Update and simplify README.md
• Remove all Binder references (there was a single env designed to run most projects, but it wasn't maintained and I guess it was no possible to run much with it)
• Show "Run notebook / Open app" links on the gallery
• Various CI updates

[github-actions]

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