Download tarballs from GitHub archive URLs instead of cloning the repo

[rafaelfranca]

This will save space by not downloading the .git folder and just getting the source code for the specified tag.

I could save 10GB in the documentation server without downloading the git repositories.

The only thing I need to double check is if the doc generation itself in the Rails repo uses the git repository in any way.

[rafaelfranca]

Confirmed that both api and guides generation work without the git repository.

[fxn]

I am fine with the patch, but if we compare

git -c advice.detachedHead=false clone -q --depth 1 --single-branch --branch $tag https://github.com/rails/rails.git $tag

with

curl -sSL https://github.com/rails/rails/archive/refs/tags/$tag.tar.gz | tar -xzf - --strip-components=1

we only save 7 MB for tag=v8.0.0 (44 MB vs 51 MB).