Add login docs

[lcjohnso]

Current State: docs currently describe login as part of create a new project tutorial, but not on its own (i.e., in a easily linkable way).

Preferred State: we add a login-specific anchor in the docs with various login methods described.

Three login options for new documentation section:

1. use Panoptes.connect(username='example', password='example')
2. use Panoptes.connect(login='interactive')
3. set PANOPTES_USERNAME and PANOPTES_PASSWORD env variables, then use Panoptes.connect() (env variables are checked as fallback)