docs: gplazma document oidc plugin

Motivation:

The `oidc` plugin currently has no documentation.  This is particularly
egregious, given the plugin's complexity.

Modification:

Add a first version of the plugin documentation, explaining how to use
the `oidc` plugin.

Result:

Hopefully a better understanding on how to configure dCache to support
oidc.

Target: master
Request: 9.1
Request: 9.0
Request: 8.2
Requires-notes: no
Requires-book: no