Update warning about write permissions

Problem/Motivation

A warning message is shown post install if $settings_dir and/or $settings_file are writeable. The message links to a very old doc, which really does not help: https://www.drupal.org/server-permissions

This warning makes sense, but updated documentation is necessary for it to be helpful.

Also, DDEV sets:
$settings['skip_permissions_hardening'] = TRUE;
so this warning is always displayed post install. Of course, there is no need to change it; it's intentional. The documentation page already has a comment from a confused DDEV user who is unsure how to resolve this.

Steps to reproduce

A few ways, but installing Drupal or Drupal CMS with DDEV is one way.

Proposed resolution

Point to more recent/relevant documentation if it exists, or write some? I'm not really sure. But should have a way for DDEV users to determine that they don't need to worry about this.

Remaining tasks

TBC

User interface changes

TBC

Introduced terminology

N/A

API changes

N/A

Data model changes

N/A

Release notes snippet

N/A