docs: Add development guideline section

[stotko]

With Sphinx, we now have much better control over the layout and structure of the documentation. Add a development section to the documentation and include the changelog, a link to the license, as well as a significantly overhauled contributing guide. The latter further includes dedicated pages for the coding style, the documentation building, and testing.

As of now, the overhauled contributing guide links to the main page. This will be updated in the future when further parts of the documentation have been revised.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (a3bd45b) 97.40% compared to head (fb30d8d) 97.40%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #398   +/-   ##
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  Coverage   97.40%   97.40%           
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  Files          31       31           
  Lines        2581     2581           
=======================================
  Hits         2514     2514           
  Misses         67       67           

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