DT-773: Clean up SAML documentation and support policy

[danepowell]

Right now BLT hosts an entire documentation page on how to set up SimpleSAMLphp: https://docs.acquia.com/blt/tech-architect/simplesamlphp-setup/

While I think it's okay for BLT to offer guidance on how to use the recipes:simplesamlphp:init command, we should definitely not be in the business of guiding or supporting configuration of SAML itself.

Specifically, we should:

1. Add a disclaimer to the recipes:simplesamlphp:init command that support requests related to SAML configuration should go to Acquia support or TAMs, not to BLT.
2. Refactor the SAML setup doc page to be specific to the recipes:simplesamlphp:init command. Anything related to configuration of SAML itself should live elsewhere in Acquia Docs.
3. Include the same disclaimer as above on the BLT SAML doc page.

Optionally, consider moving SAML into a BLT plugin, and remove it from core BLT and those docs altogether. Pros of this are that it's more clearly community-maintained and less associated with BLT. BLT becomes lighter weight for most users that don't use SAML. Cons are that the command is less discoverable (hopefully to be fixed by #3828).