Updated README.md, added support for using custom domain.

[ionutmanolache-okta]

Description

Updated documentation, and added support for using custom domains. ⚠️ This should not be merged until https://github.com/auth0/auth0-mfa-api/pull/1503 gets deployed and the FF is activated.

References

https://auth0team.atlassian.net/browse/MFA-2132

Testing - NA

• [ ] This change adds test coverage for new/changed/fixed functionality

Checklist

• [ ] I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• [ ] All active GitHub checks for tests, formatting, and security are passing
• [ ] The correct base branch is being used, if not the default branch