Most needed changes to Coq's UI page.

[Zimmi48]

@herbelin As an alternative to #218, here are a few commits which implement the most needed changes to Coq's UI page without making any radical change (in particular to the structure).

Mostly, we:

• remove excessive information that cluttered the page
• move coq_jupyter to the top of the experimental section (since it doesn't really fit the "actively used" category)
• mention the existence of experimental NeoVim support for VsCoq and coq-lsp (although we might want to remove this mention if this experimental support stops being actively maintained in the future; for now, both linked projects are actively maintained by the same person).

I'm happy to drop any commit to make the merging of this PR quick and without debate. (Each commit can be considered independent.) I have no will to spend any time debating any of this change.

Here is what the page looks with these changes applied:

For reference, here is how it currently looks: