docs: add a landing page to the documentation

[12rambau]

A pydata-sphinx-theme based documentation needs a landing page.

features

• A centered 2 column landing hero container
• left is a bit of text + title + 3 buttons (leading nowhere at the moment)
• right is a demo gif

technical addition

• hero container required extra css rules
• demo gif is now existing in both dark and light theme
• hide the index
• the hero container is mainly made of pure html to no break other potential builds (👋 .pdf)

[mariobuikhuizen]

The landing page looks much better than the site-map previously! Awesome the demo gif is now dark-mode enabled!

The buttons/links don't seem to work, but I don't know if this PR is ready yet.

[12rambau]

the button are pointing to pages that are not yet existing so I send them to "#", the PR is ready to merge !

[mariobuikhuizen]

Ok, let's wait for those pages to be available, otherwise we'll have broken links on the doc-site when we need to release from master.

[12rambau]

require #278 to move forward

[mariobuikhuizen]

What is we removed the dead links for now, then we can merge the landing page. We'll add back the links in PR's that have the actual content?

[12rambau]

@mariobuikhuizen I left the code of the buttons but made them disapear. In this configuration, wiring them once the other pages are ready will be super easy and we don't show to users dead links.

[mariobuikhuizen]

Looks good! Only needs some changes to the text.