Update contributing guidance

[MathewBiddle]

https://github.com/ioos/ioos-python-package-skeleton/blob/main/CONTRIBUTING.md

We should update this to be more specific about how folks should be interacting with these doc sites.

from @dpsnowden:

do I work on main or gh-pages? and Do I commit directly if I'm working on a site with other people or do I fork/pr?

I see two cases:

1. Those who have admin privs. (don't need to fork)
2. Those who don't. (need to fork)

We can build off https://github.com/ioos/ioos_code_lab/blob/main/CONTRIBUTING.md#building-new-notebooks

[ocefpaf]

Ideally everybody, including those who have admin rights, should fork. Sending PRs directly, without a fork, can leak secrets and/or trigger workflows that are not meant to run until the PR is merge. Like publishing a site while still iterating the commits.

Regarding the branch, that is more complicated b/c it depends on the repo. My guess is that all of IOOS documentation are in the gh-pages branch. However, they can be on main in if we wish to do so.