Website updates

[maximlt]

• Removed the link to the Introduction (home) page in the navigation bar, instead you can just click on the logo to get back to this page
• Removed some old CSS and templates, they should all be inherited from nbsite now, and update it to its latest version
• Turned off some matplotlib warnings due to missing xkcd fonts ([Bug]: Font family 'xkcd' not found warnings matplotlib/matplotlib#26282) => I tried getting rid of those in a second round but failed partially, so I re-filtered the warnings
• Simplified the tests and docs workflows
• Remove old packaging cruft
• Added link to the Blog in the navigation bar

[droumis]

This is definitely a big improvement! Apart from trying to use a dropdown to simplify the top nav, I support this being merged.

The difference in top nav between holoviz.org and the blog website should continue to be addressed on the blog repo.