First set of improvements:

* Suggest skipping SSO server installation for now, until we upgrade that codebase

* Be more explicit about the desired values for arvados_login_base and
  arvados_v1_base in the workbench installation instructions.

refs #4186
Explain the Keepstore's -permission-key-file argument and API server's
blob_signing_key configuration value.

refs #4186
Add install dependencies for the SSO server.

refs #4186
Merge branch 'master' into 4186-install-doc-improvements
Merge branch '4186-install-doc-improvements'

refs #4186