Sopel 8.0 preparations

[dgw]

We'll have to tweak quite a few things in order to launch the documentation for Sopel 8.0 properly. More changes have occurred since @Exirel assembled #38, and it seems that #39 is going to be obsolete in the face of further improvements to the Sphinx docs (sopel-irc/sopel#2497, sopel-irc/sopel#2533) that are nearing completion.

Once the outstanding "major" documentation patches are merged upstream—hopefully soon—we should place a hold on further changes to the sopel repository and spend some time getting a patch ready for the sopel.chat repo. We need to:

• build an upgrade guide for 7.x to 8.0
• remove/redirect obsolete pages to the Sphinx docs
• redirect moved pages, where feasible
• make sure links from the leftover Markdown content into /docs still go to the right place
• use the new location for core plugins (sopel.modules -> sopel.builtins)
• probably more stuff I've not remembered while writing this

The goal is to have a website patch ready for Sopel 8.0.0's release, so it can be deployed just after the package goes up on PyPI. We won't be able to update the _sopel submodule to the release commit in advance, since it won't exist yet—but we can certainly use https://preview.sopel.chat/ (the preview branch) to test things with the latest HEAD commit from upstream as needed.

[dgw]

Note: Alternatively, we can discuss simply dropping the whole Jekyll side of things. But I think it's better to get 8.0 out without delaying it too much more to wait on website changes.

We can continue pulling content from Jekyll (here) to Sphinx (in the main repo) en route to 8.1 and 9.0, plus that'll give us the time needed to figure out the best way of migrating things like individual pages for each version (the dynamically generated /changelogs/ folder, based on NEWS from upstream) and core plugin docs (the /usage/commands/ and /usage/plugin-configuration/ pages, dynamically generated from the plugins that ship with _sopel).

[dgw]

With the merge of #47, all of this should be done 🥳