Refresh the Infrastructure documentation

[jonathansick]

The /contributing/infrastructure page was ported from the original Learn Astropy Sphinx site, but nothing was done to update its content. Let's review its content and update as necessary.

One thing to keep in mind that that the scope of Learn Astropy content is now much broader; we may want to reorganize this content given the new scope.

[kelle]

done in https://github.com/astropy-learn/dev-guide/blob/main/README.md.