Support Traefik ForwardAuth without a 401 handler (#1023)

* GH-1015 Adds support for Traefik to OauthStart on '/oauth2/auth' endpoint

* Fix incorrect reference to signout path and point to signin path

- remove commented out alternative solutions and debug log statements

* Remove skip provider button check as SignIn method already does this

* Updated traefik example to match existing file configuration reference, updated tests

* Update doc and refactor nested conditional statements

* Revert code changes as static upstream provides the same functionality

- Add doc on using static upstream with Traefik ForwardAuth middleware

* update changelog

* Move the doc changes to 7.0.x versioned docs

* Re-add traefik docs update in the main docs overview.md

* add missing oauth2-proxy routing

Co-authored-by: Praveen Chinthala <PraveenChinthala@hollandandbarrett.com>