Move installation details from README to the installation guide

[kwankyu]

Currently we have two docs for installation: README.md and the Sage Installation Guide. There are severe problems:

• README is too complicated for a beginner to follow: too many details, jargons, too many branch points (links)
• There are many cross references between README and the Guide. One has to hop between the two docs.

This PR aims to make README.md a light-weight easy-to-follow installation guide for newcomers and beginners trying to install Sage to their personal computers.

See the new README:

https://github.com/kwankyu/sage/tree/p/streamline-installation-guide#getting-started

The new README does not try to be comprehensive. That is the role of the Sage Installation Guide. Materials removed from README are incorporated to the Sage Installation Guide. The Sage Installation Guide would provide all the details for users for whom the README.md is not sufficient.

The installation guide in README was tested with macOS, ubuntu, Windows WSL (ubuntu and fedora).

Along the way, I made extensive edits on the Sage Installation Guide. Most of them are non-technical.

See the new Sage Installation Guide:

https://doc-pr-38659--sagemath.netlify.app/html/en/installation/

In future, there would be little need to update the installation guide in README. Developers could focus only on updating the Sage Installation Guide as Sage evolves. This lessens maintenance burden.

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

#38676 to fix ci for doc preview

[github-actions]

Documentation preview for this PR (built with commit a5d5510; changes) is ready! 🎉
This preview will update shortly after each push to this PR.