DOC: integrated pr workflow from contributing guide into install and workflow

[story645]

PR summary

Feedback from @juanis2112 at Scipy sprints was that the workflow overview in the contributing guide was confusing because it required hopping out to install docs and workflow docs which had overlapping material, so then it was unclear what/when to come back to the contribute: workflow docs for.

This PR:

• integrates the content from the workflow section into the overlapping install/workflow docs sections.
  • A lot of the content was repeated across contribute and install/devel and was geared towards the same audience, so I mostly chose whichever I thought was more explicit/clearer.
• replaces the workflow section with a short roadmap of the "how to PR" process

attn @melissawm if I missed something here

If at all possibly would be nice to get in for PyData sprints next Tuesday, which I know is a ridiculously tight turnaround so don't expect it to happen.

Doc failure is unrelated.

[timhoffm]

I like this. Some remarks on adjacent documentation that could be improved as a result of this reordering:

• [optional] Section name "Start a pull request" (https://output.circle-artifacts.com/output/job/6d223938-8f5b-42b9-8cec-dea71c283e46/artifacts/0/doc/build/html/devel/contribute.html#start-a-pull-request): This is a bit more than just PR because it includes the setup. Is there a better name, e.g. "Start contributing"?

• in this section:

  The simplest way to do this is to use either Python's virtual environment venv or conda.

  This is biased now that there is also Codespaces. Either add Codespaces to the sentence or make it generic like "We recommend one of the following options for a dedicated development environment:

• same section, below the tab view:

  Remember to activate the environment whenever you start working on Matplotlib.

  This should move into the tabs because it does not apply to Codespaces. Then we can also spell out the activation commands for venv/conda explicitly.

I think we can get this in for tuesday 😄

[story645]

Then we can also spell out the activation commands for venv/conda explicitly.

Is already there.

This is a bit more than just PR because it includes the setup. Is there a better name, e.g. "Start contributing"?

You can't start a PR w/o the setup? Which legit is why I include the setup info there as a start. I'm not fond of of "start contributing" b/c the whole guide is about contributing & I don't want to limit "contributing" to just the types that are done via PR.