Add a page on the contribution workflow in the contributor guide

[namurphy]

During a live tutorial during Plasma Hack Week, I started an outline for a page in the contributor guide on the workflow for contributing to PlasmaPy (#1178). That only included section titles. Given insufficient time before the 0.7 release, this page got hidden temporarily in #1217. This pull request is intended to put content on the workflow page. Some of this could be adapted from Astropy's dev workflow page and other resources. We'll also need to list some resources that would be helpful for new contributors to look at while getting started.

Closes #760.

[codecov[bot]]

Codecov Report

Merging #1255 (5f602fa) into main (06e5d4e) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1255   +/-   ##
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  Coverage   97.23%   97.23%           
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  Files          85       85           
  Lines        8020     8020           
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  Hits         7798     7798           
  Misses        222      222           

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[namurphy]

I had forgotten that there was some relevant content in docs/code_guide.rst that could be copied over and adapted into this page.

[namurphy]

Superseded by #1656, largely because I forgot that I already had this pull request open.