chore(docs): Let readthedocs.org handle our theme

Fixes #6698
Merge pull request #6699 from ewinslow/docs-theme

chore(docs): Let readthedocs.org handle our theme
docs(admin): Created new admin section for admin-specific docs
Merge pull request #6701 from ewinslow/docs-organization

docs(admin): Created new admin section for admin-specific docs
fix(upgrade): Adds an admin notice when a new ElggUpgrade object is created
Merge pull request #6697 from juho-jaakkola/upgrade_notices

fix(upgrade): Adds an admin notice when a new ElggUpgrade object is created
docs(config): document custom exception handling
Merge pull request #6675 from beck24/exception-handling-docs

chore(docs): document custom exception handling
chore(i18n): configure Elgg for Transifex support

1. Add all language files to .tx/config

2. Remove comments from the top of language files.

   Transifex does not strip comments from the source files
   so they end up looking tacky when they say "English file"
   and they're actually in a different language.

3. Converted old `add_translation("en", array(...))` calls
   into the new `return array(...)` format.

   This is for a similar reason as in point 2.

Fixes #6709
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
Merge pull request #6711 from ewinslow/i18n-tx

chore(i18n): configure Elgg for Transifex support
docs(all): Cut fluff

This is an attempt at taking my own writing advice seriously.
I've gone through and tightened up the language of a bunch of the docs,
especially the first sentence of the doc which summarizes the purpose.
Merge pull request #6716 from ewinslow/docs-fluff

docs(all): Cut fluff
docs(moved): use :orphan: instead of lying toctree
docs(js): use proper title markup

The previous markup matched other heading levels within the
document itself, so sphinx was getting confused and putting
the sub-headings into the top-level table of contents.
Merge pull request #6719 from ewinslow/docs-orphans

docs(moved): use :orphan: instead of lying toctree
docs(guides): add web services dev guide
Merge pull request #6720 from ewinslow/docs-js

docs(js): use proper title markup