Keep stable and main branch docs separate

[tmandry]

We should link to latest stable docs instead of tip-of-tree where possible.

Note that this will need a redirect from the current reference URL since Glide 0.2.7+ embeds a link there.

[y3owk1n]

Just providing some references here. Some of them also do something like this:

• stable: https://docs.helix-editor.com
• master: https://docs.helix-editor.com/master

benefits: people can view what's about for stable and master configurations or guides
downside: more work to maintain maybe...

[tmandry]

Yeah I was thinking something like that. Probably I would nest stable under its own path to prevent conflicts and then have the top level page redirect.

A subdomain makes sense since I wouldn't necessarily want the homepage to follow the release schedule, and at that point it's cleanest to have two instances of Pages. That will require splitting the astro site into two.