Update sparse checkout instructions to use a fork

[mdavis-xyz]

The tips include instructions on how to do a sparse checkout. However this is how to do a checkout of typst/packages as the origin remote. Unless I'm mistaken, that's actually not a useful thing to do. What you typically want to do as a contributor is fork the repo, clone that sparsely (typically as origin), and also add typst/packages as another origin (typically upstream). If you do forget to set both of those remotes as sparse checkouts, then when you push a commit, it will download everything. So the sparse thing won't work.

I propose changing the sparse checkout code snippet to:

git clone --depth 1 --no-checkout --filter="tree:0" git@github.com:{your-github-username}/packages
cd packages
git sparse-checkout init
git sparse-checkout set packages/preview/{your-package-name}
git remote add upstream git@github.com:typst/packages
git config remote.upstream.partialclonefilter tree:0
git sparse-checkout reapply

(I'm not sure if all those steps are necessary.)

[elegaanz]

That makes sense. But yes, I don't think the last command is necessary. It would be great if you could check, and then make a PR to change that if you are willing to.

[mdavis-xyz]

I've just tested. The last line is not necessary, but adding git checkout main is necessary.