Make index page match toctree structure

[sneakers-the-rat]

Part of: pyOpenSci#181

As always, with any of my comments or reviews, take this or leave it! i won't be offended if you don't like these changes :).

Currently it's not obvious that the content on the index page is related to the tabs at the top of the page. This makes the overall structure of the site a bit bewildering - I'm not quite sure if this index is unrelated to the tabbed sections, and it's difficult to scan through the boxes in the grid quickly.

This PR reorganizes the index page to match the toctree, adding titles and links to other subpages. It might be a few too many links without text, we can take out some of the docs links, but hopefully this makes the different groups a bit clearer.

I changed the order of the toctree to put packaging before documentation because the blocks on the index for packaging came first, but that can also go back, i just think they should match the order!

before:

after:

[lwasser]

once i merge the open PR - let's rebase this against main and then look at the build. i think that the structure IS confusing now and this will make it better. let's just add it as a separate pr if you are ok with that! thank you so so much for this @sneakers-the-rat

[sneakers-the-rat]

sounds gud :)