Document password validation changes

I spoke with and he advised to create a ticket surrounding documenting password validation changes.

As part of the work on LEARNER-4674, edX switched from using a custom built password validator to Django's framework of password validation. This involved creating a new Configuration value, AUTH_PASSWORD_VALIDATORS, a list of Django and edX-created validators. To facilitate code cleanliness, we have removed unnecessary Configuration values preferring to specify the values in AUTH_PASSWORD_VALIDATORS. The removed variables are PASSWORD_MIN_LENGTH, PASSWORD_MAX_LENGTH, and PASSWORD_COMPLEXITY. The Pull Request adding the new value can be found in https://github.com/edx/configuration/pull/4810/files and the Pull Request removing the unnecessary values can be found in https://github.com/edx/configuration/pull/4811/files. Edx-internal and edge-internal have similar Pull Requests to utilize the new password validation framework.

Including a new validator involves creating it somewhere (see below) and adding it to the list in AUTH_PASSWORD_VALIDATORS.

Writing a new validator looks like:

Please let me know how else I can help!