move pytest-dev contributing docs to pytest-dev/meta

[graingert]

I think this part of the documentation makes sense in pytest-dev/meta:

https://github.com/pytest-dev/pytest/blob/bc0020ad96f1d4b831655cdaa6b535f86ee15367/CONTRIBUTING.rst#submitting-plugins-to-pytest-dev

[graingert]

also we should clean up the purpose a little:

currently the docs note that the purpose is:

Sharing some of the maintenance responsibility (in case a maintainer no longer wishes to maintain a plugin)

but another core contributor had some concerns with that wording, paraphrasing, that the maintenance burden would fall on the core team.

my response: Maybe that should be in the purpose statement of pytest-dev? I see the org as a place where a new maintainer can be given access to a repository if the original maintainer goes AWOL
I see that being more useful if adding the pytest pypi user account to owners is a condition of membership

for example, Eg the pypa maintainer-ship handover is 1 year of radio silence, the pytest one could be 1 month of AWOL, or 1 week of AWOL if it's incompatible with pytest stable. For example, passing the maintainership to someone with a patch to make the project work again

[The-Compiler]

I think this part of the documentation makes sense in pytest-dev/meta: [...]

Disagreed - I think it makes sense to keep it as part of the pytest documentation, where people would expect to find it; rather than in a place nobody will look.