split out doc rules to a separate page
moved astropy.sphinx.ext docs to docs section
Minor text fixes
adjust edit_on_github docstring to be in line with other extension docs
automodsumm doc tweaks
Added blurb at beginning of docguide
typo fix in edit_on_github
various title wording and ordering tweaks in developer docs
Reorganized building/releasing into three sections
Title change for leading dev section in docs
moved vision from toc to inline reference
Typo fixes pointed out by @mdboom
Fixes suggested/inspired by @iguananaut
Changed _ASTROPY_SETUP_ checking as per @iguananaut's suggestion in #922
Merge pull request #922 from eteq/add-sphinx-ext-docs

Clean up dev doc organization, especially sphinx extensions