PR: Update install guide to reflect standalone installers and other details

[CAM-Gerlach]

Pull Request

Pull Request Checklist

• [x] Read and followed this repo's Contributing Guidelines
• [x] Based your PR on the latest version of the correct branch (master or 4.x)
• [x] Checked your writing carefully for correct English spelling, grammar, etc
• [x] Described your changes and the motivation for them below
• [x] Noted what issue(s) this pull request resolves, creating one if needed

Description of Changes

Adds and updates a number of things in the install guide to reflect the new standalone installers and other issues:

• [x] Update mentions of Anaconda being "recommended" on non-Linux platforms and add admonitions reminding users that the installers are
• [x] Add information on how to run and update standalone installers
• [x] Link to FAQs on installing packages and using existing environments with standalone Spyder
• [x] Clarify what lite installers are and their benefits/drawbacks
• [x] More strongly and generally warn users away from using distro packages
• [x] Add note on necessary fix for pip installs to work under some Linux distros
• [x] Fix layout of installer buttons on mobile
• [x] Fix and improve description of WinPython
• [x] Fix a few other minor things

Issue(s) Resolved

Fixes #201 Fixes #237 Fixes #251

[CAM-Gerlach]

Also, related to this but for a separate PR, we should consider backporting the FAQ question on installing new Python packages with standalone Spyder (added in PR #259 and improved in subsequent PRs) to 4.x, since it is fully applicable to both versions and so we can directly link the FAQ questions here instead of just the section, without causing merge conflicts.

[CAM-Gerlach]

Investigating the linkcheck failure, it is apparently due to the Gentoo package site running into issues, which are apparently persisting when I try to access it now (the homepage loads, but no packages are found, and the sub-links down work). If this continues once this is ready, I'll either add an exclusion or remove the link.

[CAM-Gerlach]

@mrclary Do you mind checking over the standalone installers section and double-checking there isn't anything major amiss, and if so marking the conversations as resolved and dropping an approval on here? Thanks!

[CAM-Gerlach]

Thanks @mrclary . Good points; with this change, we do refer to both of those points in the Using Spyder from the standalone installers section and link to the FAQ section that specifically address how to do so.

[CAM-Gerlach]

Thanks @mrclary , merging this.