Make first draft of Raven public docs

Provide a first draft of this documentation. Estimated as a spike so that we evaluate where we are after some reasonable time.

The documentation should include:

• The purpose and scope of Raven.
• The list of supported protocols (SAML 2.0, OAuth2 via G Suite@Cambridge).
• A feature matrix comparing them.
• Instructions on registering an SP for SAML 2.0 and OAuth2 via G Suite@Cambridge.
• Examples of Apache configuration for SAML 2.0 and OAuth2 via G Suite@Cambridge.
• Examples of using our proxy images for SAML 2.0 and OAuth2 via G Suite@Cambridge.
• User-focused pages explaining the UX of SAML 2.0 and OAuth2.