The TOKEN_EXPIRATION_TIME is not defined in settings.py

[lubosmj]

We should add TOKEN_EXPIRATION_TIME to settings.py and ensure it is easily discoverable.

I see only one occurrence of this setting being mentioned along the lines in the documentation:

pulp_container/docs/admin/learn/authentication.md

Line 115 in 1a8fe63

via the setting `TOKEN_EXPIRATION_TIME`. The default expiration time is `300` seconds.

Inspired by #1592.

[git-hyagi]

In addition to that, we could create a doc mentioning the TIMEOUTS in general (like API_APP_TTL, PULP_GUNICORN_TIMEOUT, TOKEN_AUTH_TTL, etc).