improve developers docs

[giovp]

this PR does:

• splits the developers_docs.md in:
  • developers_docs.md titled Using this template which explains how to go from running cruft ... to making having a functional github repository.
  • contributing.md titled Contributing guide which explains how to contribute to the repository (from docs building to publishing)
• extend instructions on codecov and add example on manually publishing a python package

this PR does not:

• add high level tutorials on python packaging (to be done in separate PR).
• documentation on github actions (to be done in separate PR).
• adding the automated publishing discussed in Add action for publish/upload on PyPI #88

actually close #119

[github-actions]

A PR has been generated to the instance repo:
scverse/cookiecutter-scverse-instance#33

You can check out the PR to preview your changes
in an instance of the cookiecutter template. The PR will be kept in sync with
this PR automatically.

[grst]

LGTM!

[ivirshup]

Looks good. A few minor comments, I may also just fix these myself once I'm off the train and back to wifi

[ivirshup]

I think we also discussed changing the name of the developer docs, since the contributing guide is also developer docs.

Maybe something like: "managing this repository"?