I'm not sure if this should be merged yet, since the "discussion" on sage-devel is ongoing.

I'd say, let's just go ahead. Haven't seen any reaction in the thread (other than yours)

Cloning the repo uses 1GB of disk (without any upstream packages downloaded.) Using the tarball seems more economical to me in this case. Downloading a tarball of the GitHub repo (which is not advertised anywhere) is just 28MB for me.

Anyway, I don't really mind much either way. If you guys think this is a good idea, go ahead.

Cloning the repo uses 1GB of disk (without any upstream packages downloaded.)

no, it's "only" 615 Mb, and over 60% of it is .git/

$ git clone -c core.symlinks=true --origin upstream --branch develop --tags https://github.com/sagemath/sage.git
...
$ du -h sage/
...
615M	sage/
$ du -h sage/.git
388M	sage/.git/objects/pack
...
390M	sage/.git

Adding --filter=blob:none, as recommended in https://github.blog/2020-12-21-get-up-to-speed-with-partial-clone-and-shallow-clone/ - results in only 141Mb in sage/.git, and 366M for the whole sage/.

Perhaps this is, disk space and bandwidth-wise, a better advice than the one in the PR, to use the latter.

Using the tarball seems more economical to me in this case. Downloading a tarball of the GitHub repo (which is not advertised anywhere) is just 28MB for me.

Anyway, I don't really mind much either way. If you guys think this is a good idea, go ahead.

Done in 04145de

Also need to link to git install instructions (similar to #30201)

Edit: Done in 8a1a459

Documentation preview for this PR (built with commit 3341cdb; changes) is ready! 🎉

Thanks for the review.