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
If that's the case, it shouldn't be under a blank "Mac" heading.