Document the `/register` OAuth2.0 endpoint

[ColinFrick]

Document the /register OAuth2.0 endpoint

Problem

The /register endpoint is not documented: https://fusionauth.io/docs/v1/tech/oauth/endpoints

Solution

Add documentation for /register

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[mooreds]

Per #686 (comment) the registration URL is always the same as the login URL except with register instead of authorize.

[robotdan]

Internal:

Let's review and see if we can just add this to the OpenID Configuration endpoint response.

[mooreds]

Is this going to be just a doc update, then?