Shrink header, add get in touch & update to rocq-waterproof

[DikieDick]

Changes

• Shrunk the "Waterproof" header, as I think it takes up a lot of vertical space. (Can be easily reverted)
• Added a get in touch section with a link to the Waterproof Zulip.
• Changed the 'coq-waterproof' link to 'rocq-waterproof'.

Ideas

• Instead of the "GitHub source" we should maybe create a new webpage detailing the projects under the 'impermeable' organization (with their respective goals, features, etc.), as we currently advertise just a subset in this section.