Add document/page with detailed explanation about Login Server

[stefandesu]

From #27:

Add an end-user friendly help page that explains the purpose and functionality of login-server and single-sign-on with basic concepts such as

Identity providers (e.g. ORCID)
Identities
Client applications (Cocoda)
Sessions
Service providers (e.g. jskos-server, Wikidata-write-access...)
API

Either a document inside the repo (that could then be linked from the /help page), or a separate sub-page.