Clarify SSO docs about sentry-docs

[dcramer]

This is resolved with v8 of Sentry as those docs no longer exist and are replaced with this repo + docs.getsentry.com

[DominicWatson]

I personally don't think that removing the docs for the next version goes far enough. The difficulty is that an online search will intimate to curious minds that built in SSO is possible (historic records, etc.). Removing the docs on SSO will do nothing to deny that and leave unfortunate souls with the impression that "the new docs are lacking". This is what happened to me, and I found bits of information that suggested it should work but none to say that it absolutely isn't possible (in an out of box kind of way).

Instead, create a page in the documentation for Single Sign On that has:

1. The fact that there is no out of box SSO solution for On-Premise solutions
2. The fact that SSO options are available in the hosted version (if that's true)
3. The fact that the authentication API lets you roll your own with links to developer docs
4. The fact that you can support customers needs commercially
5. The fact that third party authentication plugins may/do exist