Document JWT and impact on OIDC

[mmattel]

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