Move section on assigning environments to its own page
Move info on enabling directory environments to its own page
Move stuff on creating environments into its own page; update environment.conf page

This also moves setting descriptions to the environment.conf page, since the
settings I was linking to had big and non-applicable deprecation warnings.
Add new pages to navigation, update info in modulepath page
Re-do the environments.html landing page
Add tips on environment timeout to environment.conf page
Merge branch 'direnv_adjustments', resolving DOC-1002, DOC-1003, DOC-1057, DOC-1078