Fix(mail): Add debug log when creating a user and add documentation about email rate limit

[jonathan-marmelab]

Problem

When mail rate limit is reached, it is impossible to create a new user

Solution

Add documentation on how you can set up a custom SMTP server to circumvent the limit.

How To Test

N/A

Additional Checks

• The documentation is up to date
• Tested with fakerest provider (see related documentation) N/A