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