docs(appendix): move about => appendix; add contribute guide

Also:
 * Left notices for moved docs so we don't break inbound links
 * Added section for moved docs to make sphinx happy
 * Moved contributor guide to its own section (to be expanded)
docs(contribute): expanded contributors section

Now in 4 sections, with more to be added:
 * docs (how to write good documentation)
 * issues (reporting bugs and feature requests)
 * code (submitting pull requests)
 * money (becoming a financial supporter)
Merge pull request #6713 from ewinslow/docs-contribute

Contributor Guide, Appendix, and link BC