Roles: Merge documentation infrastructure and testing infrastructure

[pllim]

Documentation and testing infrastructure are closely related and both are actually managed by the same set of people in reality. say, it is tough to maintain pytest-astropy without also touching sphinx-astropy since doctest tie them together. And both documentation and tests are done in the same manner (e.g., the CI checks for PR).

If simply merging them is too controversial, we can also try to have each of them as "sub-role" but shared by the same set of people. 🤷

Affected people in the merged listings:

• @saimn
• @pllim
• @astrofrog
• @bsipocz

TODO:

• Link to chart from Infrastructure/DevOps needs attention astropy-project#118 once it is updated and in the repo.
• Add sub-roles for the many things that we actually work on

[astrofrog]

That would be fine by me

[pllim]

Given @hamogu's mention of "phone book," I could also expand this PR to add all the sub-roles for infrastructure. I mean, right now they are all the same people, but I guess it is not guaranteed to remain so when/if we finally have more staff to help out.

Please see astropy/astropy-project#118 .

[hamogu]

This will be rebuild as a separate PR, where infrastrucure roles will be specified in more detail