Improve documentation on configuring plugin

Docs are not too clear on how to configure the keys. We should mention specifically that they should be set in `wp-config.php`, and could also mention using `wp-cli` to set it.

Docs are also not too clear on how to set `oidc_clients`. It's suggested to use `functions.php`, which is not ideal because it's tied to the theme. We should probably suggest using a plugin (or probably even an mu-plugin) to configure the `oidc_clients`.

Start by locating the existing documentation sections covering key configuration and oidc_clients setup. Update them to identify wp-config.php, mention wp-cli, and recommend a plugin or mu-plugin instead of functions.php; done means all three configuration paths are explained clearly.

Written by the indexing model from the issue text.