docs: Combine dev setup docs, refactor organization
    
    This commit does two things:
    
    * Combines the two development set-up guide docs into one
    * Refactors the combined page for better organization
    
    I thought it was confusing to have two pages, since they mostly repeated
    the other pages. This makes it easier for us to maintain one and only
    one source of truth for setting up and developing on Fedora Happiness
    Packets.
    
    I also refactored the combined pages and reorganized parts to make it
    easier to read. It is a fairly long page, so I used some more
    ReStructuredText / Sphinx magic to improve readability where I could.
    
    This page also now conforms to most of the Sphinx docs style guide too:
    
        https://documentation-style-guide-sphinx.readthedocs.io/
    
    Some of these, like filenames, I intentionally ignore because I think
    they are quite silly.