Add documentation on password requirements

Blocks weak passwords on sign up or password ch... (!86310 - merged) is introducing a disabled-by-default feature flag which prevents users from choosing weak passwords. We should document how this works.

The following discussion from !86310 (merged) should be addressed:

@nmalcolm WDYT of adding a docs update to: https://docs.gitlab.com/ee/user/profile/index.html#change-your-password. Probably just adding: https://docs.gitlab.com/ee/development/documentation/versions.html#add-a-version-history-item.

@eread There are a few places it could go and it might make sense to rationalize it into a single location?

To keep this MR simple, how about we leave it as is and create a new issue under Prevent users from choosing weak passwords (&8139) to discuss where to put docs and have that as a blocker for enabling the feature flag on .com?

My proposal would be a new page like https://docs.gitlab.com/ee/security/choosing_a_secure_password.html which

  • Describes the constraints applied by Block Weak Passwords (and whether it's enabled or not / its version history)
  • Describes .com's additional password constraints (migrated from the gitlab_com page, which is updated with a link to this new page)
  • Links to constraints that might optionally be applied by administrators (Sign Up Restrictions and Custom Password Length Limits)

And we update https://docs.gitlab.com/ee/user/profile/index.html#change-your-password (and potentially the sign in / sign up page or the weak password error itself) to link to "Choosing a secure password".