Describe strong passwords and GitLab requirements

What does this MR do and why?

Resolves Add documentation on password requirements (#373225 - closed)

Password requirements and configuration are scattered across a number of documentation pages. This creates a user-focussed page on choosing a password which summarises and links off to those other pages.

It also serves to describe the Block Weak Passwords feature, moving it from its temporary location on the Profile Index page. This page was not ideal as weak passwords are blocked whenever a password is set or changed, including by administrators.