Add dev docs about user profiles and authentication

Closes #463
Closes #451
Signed-off-by: David Mehren <git@herrmehren.de>

docs/content/dev/authentication.md

@@ -0,0 +1,87 @@
+# User Profiles and Authentication
+Each user in HedgeDoc 2 has a profile
+which contains the following information:
+
+- username (`janedoe`)
+- display name (`Jane Doe`)
+- email address, optional (`janedoe@example.com`)
+- profile picture, optional
+- the date the user was created
+- the date the profile was last updated 
+
+HedgeDoc 2 supports multiple authentication methods per user.  
+These are called *identities* and each identity is backed by an
+auth provider (like OAuth, SAML, LDAP or internal auth).
+
+One of a users identities may be marked as *sync source*.  
+This identity is used to automatically update profile attributes like the
+display name or profile picture on every login. If a sync source exists, the
+user can not manually edit their profile information.
+If an external provider was used to create the account,
+it is automatically used as sync source. 
+
+The administrator may globally set one or more auth providers as sync source,
+e.g. to enforce that all profile information comes from the corporate
+LDAP and is the same across multiple applications.  
+If global sync sources exist, new accounts can only be created using
+these auth providers. The auth provider that was used to create the account
+is automatically set as sync source and cannot be changed by the user.
+This effectively pins the account to this provider.
+
+## Example: Corporate LDAP
+The administrator wants to allow users to log in via the corporate LDAP
+and Google. Login must only be possible for users present in LDAP and 
+all users must be displayed as they are in the LDAP.
+
+The admin therefore sets up two login providers:
+
+- corporate LDAP, marked as global sync source
+- Google OAuth login
+
+If a new user tries to log in via Google, they will not be found in the
+database. The frontend detects that a global sync source exists and
+suggests logging in via LDAP first.
+
+After a new user created their account by logging in via LDAP, they can use
+the 'add a new login method' feature in their profile page to link their
+Google account and use it to login afterwards.
+
+## Example: Username Conflict
+HedgeDoc is configured with two auth providers.
+
+- A user logs in using auth provider A.
+- The backend recieves the profile information from provider A and notices that the username in the profile
+  already exists in the database, but no identity for this provider-username combination exists.
+- The backend creates a new user with another username to solve the username conflict.
+- The frontend warns the user that the username provided by the auth provider is already taken and that another
+  username has been generated. It also offers to instead link the new auth provider (in this case A) with the
+  existing auth provider (in this case B). 
+- If the user chooses the latter option, the frontend sends a request to delete the newly created user to the backend.
+- The user can then log in with auth provider B and link provider A using the "link auth provider" feature in 
+  the profile page. 
+  
+### Handling of sync sources and username conflicts
+
+#### Global sync sources
+If at the time of logging in with auth provider A, *only* A is configured as a *global* sync source,
+the backend cannot automatically create a user with another username.
+
+This is because:
+
+- Creating new accounts is only possible with a sync source auth provider.
+- Setting an auth provider as sync-source entails that profile information the auth provider provides must be
+  saved into the local HedgeDoc database.
+- As the username the auth provider provides already exists in the database, a new user cannot be created with 
+  that username.
+  
+In this case, the frontend should show the use a notice that they should contact an admin to resolve the issue.
+
+!!! warning
+    Admins must ensure that usernames are unique across all auth providers set as a global sync source.  
+    Otherwise, if e.g. in both LDAPs configured as sync source a user `johndoe` exists,
+    only the first that logs in can use HedgeDoc.
+
+#### Local sync sources
+If auth provider A is configured as a sync source by the user, syncing is automatically disabled, and a notice is shown.  
+Re-enabling the sync source is not possible until the username conflict is resolved, e.g. by changing the username
+in the auth provider.

docs/mkdocs.yml

@@ -11,6 +11,7 @@ nav:
       Toolbar: interface/toolbar.md
   - Developer:
       - '2.0 Development': dev/2.0.md
+      - 'User Profiles & Authentication': dev/authentication.md
       - Configuration: dev/config.md
       - 'Documentation': dev/documentation.md
   - FAQ: https://hedgedoc.org/faq