Update manifesto with current state of affairs.

[Zimmi48]

• Link to Hydra Battles and cie (cc @Casteran).
• No more editorial board (didn't materialize).
• Reorganize and extend FAQ (new questions pointing to the contributing guide).
• Fix links to issue templates in the contributing guide.

[Zimmi48]

@anton-trunov I had requested a review from @palmskog, but he seems unavailable. Would you mind reviewing / approving this PR?

[anton-trunov]

Looks good to me, Théo!

[Zimmi48]

Thanks!