add appendix with topic pathways

[willingc]

Related to #40, #120, #189

This PR adds an Appendix that groups the individual document pages into logical subtopics. The goal of this PR is to gather topics together to help:

• consolidation and refactoring of sections that only have one paragraph
• deduplicate redundant content

My hope is that this can be a first pass and that we can iterate on this in future PRs.

[ezio-melotti]

This looks great, and might even be a first step in reordering the pages in the devguide (something I wanted to do as part as #120), eventually making this appendix obsolete.

One distinction I want to make that is missing from this, is that contributors should read setup.rst and pullrequest.rst; core devs should read setup.rst, pullrequest.rst, and committing.rst.
All other pages don't necessarily have to be read, but consulted if and when the need arises.

[willingc]

Hi @ezio-melotti. Glad you like it. My goal was to make this a starting point for refactoring/deduplicating content, and I agree it may be unneeded after that.

I've gone ahead and added the sections you mention and put them in a note box. I'm going to go ahead and merge. Feel free to iterate on another PR. Thanks.