ocis: Document JWT and impact on OIDC

Migrated from owncloud/docs-ocis#349 — opened by @mmattel on 2023-01-13.
Original: https://github.com/owncloud/docs-ocis/issues/349

JWT (Java Web Tokens) need more descriptions when it comes to OIDC. This is because managing the optional KID header can return an error like failed to verify access token: the JWT has an invalid kid: could not find kid in JWT header. In such a case you need to set PROXY_OIDC_ACCESS_TOKEN_VERIFY_METHOD=none.

See the original issue where this came up:
ownCloud: Enable OpenID Connect Authentication and
What's the meaning of the "kid" claim in a JWT token

There are for sure more JWT examples or things that need documentation to guide customers into the right direction when setting up ocis.

• We need infos from development to see where this goes as docs does not have the experience on this topic

@dragotin as discussed
@micbar fyi

Start with the migrated issue and its linked OIDC and JWT references, then review the existing ocis documentation for authentication setup. Ask development to clarify the supported KID and access-token verification behavior; done means the relevant JWT/OIDC cases and configuration guidance are documented for customers.