Add trusted providers section to OAuth documentation

[martinoak]

In release 6.24.2 and 5.74.1, a new trusted_providers config has been implemented. Since we use custom OAuth2 provider, people couldn't login into the Control Panel without adding the trusted_providers array with our provider in it.

Feel free to rewrite the docs, I just wanted to pointed out that this section is missing in the docs.

Related: #1946

[jasonvarga]

@martinoak Logging in should've required no changes.

The only thing that would've changed would have been trying to logging into an existing Statamic account with an OAuth provider that was not already linked to the Statamic user.

Are you using v5 or v6?

[martinoak]

@jasonvarga still on v5, we use our own company provider, but since the user wasnt even in the file that stores oauth id with our provider's identifier. When I added trusted_providers array key, everything worked even on clean installations.

[jasonvarga]

But when the user tried to log in with your company OAuth for the first time, was there already a Statamic account with that same email?

[martinoak]

Yes, otherwise "Unauthorized" message would appear.

[jasonvarga]

Cool cool, thanks. Addressed in c3b2919 for the v5 subdomain.
Equivalent for v6 coming shortly.