Markdown fixes for developer setup

This branch contains several Markdown-related fixes for the developer setup instructions, including:

1. Proper order of headings
2. Alignment of pre blocks within bulleted lists
3. Unordered list under Fedora changed into an ordered list (as it contains steps
4. Removal of `postgresql` and `postgresql-libs` as they're pulled in as dependencies anyway — this allows all the lines in that pre block to be shorter (as comments are aligned), so that lines do not wrap
I think this one should not change. Setting up got and github is common to all OSes, and is not a subsection of the Mac section.