[Feature] Document authentication methods and related authenticated-user details

[fmigneault]

Although Magpie offers multiple external provider login methods, these are poorly documented, making it hard for devs and users to even know about them (relates to #173).

Also, even for "internal provider", it is not clearly documented how to authenticate one-self. Some details can be inferred from OpenAPI definitions, but one has to dig quite a lot to find such details. These details should be provided more explicitly in docs, such as :

• how cookies are handled?
• what is the auth_tkt (or alternative value via config)?
• are Authentication Bearer token handled? (relates to Support basic authentication for ziggurat? #255)
• etc.

Given that authentication is such a main component of interactions with Magpie, appropriate details should be provided, in a dedicated section in the docs.

These details should also consider changes in #340 which provide new functionalities based on non-admin level users.
Doc should be provided to better explain contextual request behavior (ie: when Logged User is applicable to views).