Release integration for Sopel 8.0.0

[dgw]

This PR will serve as a pre-release integration point for all the moving parts that come with the new Sopel 8.0.0 release:

• Refreshed docs from upstream (the basis for this PR, once I get it working—see below)
• Upgrade guide (Sopel 8 upgrade guide #46)
• Modified redirects & links (Update links to new documentation #38)

I'm opening this in a non-working state at first. Updating the _sopel/ submodule currently raises an unhelpful error in Netlify deploy logs as shown below, and I wanted to move debugging that out of #46 where I started it.

Cryptic Netlify error

Generally, it also just seems like a good idea to have an intermediate stop where all of the 8.0.0 updates will come together before they hit the site's production domain. Fixing unexpected build errors is less stressful on a PR branch than on master. 😁

Once _sopel/ is sorted out, other PRs about 8.0.0 prep can be edited to use this branch as their base instead.

[dgw]

Ooh, this is gonna be a fun puzzle. We are going to need updates from #38 to build successfully with the updated _sopel/ submodule, and they will have to be done together. Otherwise Jekyll's link tag will fail the site build:

10:06:47 AM:   Liquid Exception: Could not find document '/docs/cli.html' in tag 'link'. Make sure the
               document exists and the path is correct. in /opt/build/repo/_usage/command-line-arguments.md

I knew reorganizing pages would come back to bite us one day! 😁

Edit: Merging #38 and applying a bit of extra fixum on top yielded a successful build. It might still contain some broken links that don't use the {% link %} Liquid tag, and/or some missing redirects, but it works.

[dgw]

I misclicked and merged #38 direct to master (whoops), after cherry-picking a redirection update that breaks the build until the changes come in from upstream. So I rebased this branch onto master and pulled #46 in to do final testing.

[Exirel]

Sounds good.