Revise Install Guide to provide better, more up to date guidance

[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

In response to the detailed user feedback on #334 , this PR:

• Expands and updates the guidance in the installation guide surrounding the standalone installers and Anaconda/Miniconda
• Adds more details about the standalone installers, their limitations and how to work around them,
• Adjusts the recommendations accordingly
• Adds a description of installing via Miniconda rather than Anaconda, and guides users toward it
• Makes other related smaller improvements

Additionally, I've revised and updated the other sections in the install guide to ensure they are complete, up to date, and easy to follow.

Live preview of updated install guide

Issue(s) Resolved

Fixes #334

[CAM-Gerlach]

In the process of responding to @jitseniesen 's comments, I ended up noticing a bunch of things that were out of date, suboptimal or could be clarified/expanded in the other sections too, so I went ahead and revised them here as well. Due that, I've manually applied the remaining relevant comments from @ccordoba12 with various fixes, updates and adaptations, since most of them are now out of date.

Should we recommend Miniforge / Mambaforge instead of Miniconda?

I thought about that before, actually, but wasn't sure about it at the time.

On one hand it would avoid needing to tell users to explicitly specify the Conda-Forge channel when installing Spyder, and avoids problems when they update it and forget to specify it (but that can be fixed by adding a conda config --env --add channels conda-forge command). Mamba would also speed things up, (but libmamba is now available as a solver for conda thanks to Felipe's work, and users may not be as familiar with it).

On the other hand, we certainly still want to mention Miniconda, since as far as I'm aware it is still a lot more popular than Miniforge/Mambaforge, has a much friendlier website and documentation, and we want to support people with their existing environment.

With that in mind, I rewrote and updated the Anaconda/Miniconda section to refer to Conda installs more generally, mentioning the various distributions including those. This also inspired me to update the other sections as well, as mentioned above.

[CAM-Gerlach]

@ccordoba12 any chance you could take another look at this? Also, @jitseniesen , would you to review this as well?

[CAM-Gerlach]

Thanks all for your great feedback! I did a bit of rebasing to clean up the commits for merging and give you both credit for your suggestions; otherwise looks like this is good to go (since we haven't heard from Jitse in a bit, and his feedback was implemented), and so I can switch to squash merge only to make sure that #331 isn't accidentally non-squashed with the large images/GIFs.