Update links to new documentation

[Exirel]

This is related to PR 2257 of sopel-irc/sopel repo.

I tried to find all the possible links, so I may have miss some. The main two changes being:

• db.html is now in package/db.html
• config.html is now in package/config.html and the core section has its own file too

I also added a few links because I think they are appropriate and in-line with a more user-friendly approach to documentation.

[dgw]

Is this meant to add redirects from the old to the new paths? I don't see those changes.

I also have to make extra sure that I don't merge this before the docs are updated. So if you don't mind, I'll convert it to a draft. 😁

[Exirel]

Oh right, I forgot about redirect!

[Exirel]

Redirects done. I think it should be enough. I'm not sure, it's always a bit hard to track everything: I looked at the files I moved, not the newly created.

[dgw]

LGTM, though I will have to hold off on actually merging this until the new Sopel version is out. Expect the base branch of this PR to be changed later, when there is a "Sopel 8" branch to merge it into with other updates (new migration guide etc.).

[Exirel]

As expected!

[dgw]

Going to put all this into #47 after updating some of the underlying config/tooling (like the outdated Python version Netlify was configured to build with when this PR started).

[dgw]

…or I'll forget to change the base and merge to master instead. Whoops.