Streamline and update installation instructions

[half-duplex]

Make the install instructions more streamlined for common goals and clarify which methods are recommended and recommended against. Also removes the ssl backports thing and the system requirements page, since pip installs all of that automatically, and if you're not using pip you should know how to handle it yourself.

• [ ] Do we still use/need sopel.py on windows?
• [ ] Can I simplify it further by pushing venvs? System-wide pip is disgusting and shouldn't be used except by people building docker containers, and they'll know how to adapt the instructions. Only concern is windows where I have no idea how any of this works.
• [x] Todo: Plugin install section

[dgw]

Do we still use/need sopel.py on windows?

sopel.py no longer exists in 8.0-dev (sopel-irc/sopel#2328, sopel-irc/sopel@866c339c7285a0d729d12b97b2cadd6e8e754d4b)

Can I simplify it further by pushing venvs? System-wide pip is disgusting and shouldn't be used except by people building docker containers, and they'll know how to adapt the instructions. Only concern is windows where I have no idea how any of this works.

I would personally leave both system-wide and venv methods documented, because in some cases setting up a venv will be a waste of time, and/or overwhelming to new users who've never run Python apps before. Global installation shouldn't be a problem if Sopel is the only (or one of very few) Python packages the user wants to try, and I would argue that it's likely someone already running multiple Python apps already knows about venvs.

No CI tests are run on Windows, so I think we're still at an "if it works, great" state. You can also leave the system-wide pip method for Windows, and/or suggest putting Sopel inside WSL (which uses the Linux instructions for whichever distro is run).

[dgw]

There's been quite a bit of work upstream on the Sphinx documentation, including a comprehensive guide on installing and running Sopel. See sopel-irc/sopel#2497 and sopel-irc/sopel#2533; we should simply remove and redirect duplicated pages from this website repo as part of the 8.0 launch process.

[dgw]

Merging of sopel-irc/sopel#2533 obviates most of this, so I'm going to close it out. venv-related follow-up belongs in the upstream docs now, if you're still motivated to make the installation guide cover that usage. (There's likely a third-party venv guide we could link to and reference, rather than reinventing that wheel.) These outdated pages should be redirected/removed as part of #38 and/or #45.