Docs: Improve guidance around authentication/OAuth [UXR]

[josmperez]

Incorporate feedback such as this:

• Better explain how to use routes in authentication.
• Better explain OAuth flow.
• Add an example of OAuth with a backend API to show how to store secrets, and how to refresh an OAuth token when it expires.
• How to figure out which fields are secrets.

Note that we don't want to repeat OAuth documentation.

[andresmgot]

Some thoughts:

• Better explain how to use routes in authentication.

FWIW, using "routes" and the plugin proxy in general is not something we want to promote, more like a legacy feature we don't plan to keep working on.

• Better explain OAuth flow.

The current documentation that we have for this:

• Using routes / plugin proxy: https://grafana.com/developers/plugin-tools/how-to-guides/data-source-plugins/add-authentication-for-data-source-plugins#add-an-oauth-20-proxy-route-to-your-plugin
• Forward Grafana OAuth login info (backend plugins): https://grafana.com/developers/plugin-tools/how-to-guides/data-source-plugins/add-authentication-for-data-source-plugins#forward-oauth-identity-for-the-logged-in-user
• There is an experimental feature, added by @yesoreyeram to add OAuth capabilities to the default HTTP client. I haven't seen this be used for any plugin so maybe you can ping him for some docs about how to use it.

• Add an example of OAuth with a backend API to show how to store secrets, and how to refresh an OAuth token when it expires.

I don't think this is something managed by plugins. You can refer to the general auth docs from Grafana.

• How to figure out which fields are secrets.

I also think this is just generic auth knowledge? Nothing related to plugins.