Add user env doc files - Githubissues

This documentation PR:

Re-adds user environment files that may have been mistakenly removed in #863. I converted the .rst files to .md and fixed up the links. Now The user environment heading has sub-content again. This issue may have occurred because env is in the .gitignore, so I renamed the folder to user-env to prevent this in the future. (EDIT: redirects are added)
A caption was added for the Authentication section in the navigation.
add_users.txt and add_packages.txt were changed to .md and the provider installation pages were updated to reflect this change.
A few broken links have been fixed.

As a result, some missing documentation has been filled in and the How-To Guides' left-side section navigation has the correct structure.

jupyterhub / the-littlest-jupyterhub