Update documentation for Default admin theme

Problem/Motivation

Currently, we do not have Gin documentation migrated to https://www.drupal.org/docs/core-modules-and-themes/core-themes .

In the description in default_admin.info.yml there are two links to the old Gin documentation, which are not correct with regards to the Default admin theme. For example there is a link to https://www.drupal.org/docs/contributed-themes/gin-admin-theme/custom-th... , which mentions a possibility to Extend via gin-custom.css, but that is not true for Default admin (the filename is different, etc).

So I think we need to:

1. Copy (and update) relevant documentation from https://www.drupal.org/docs/contributed-themes/gin-admin-theme to https://www.drupal.org/docs/core-modules-and-themes/core-themes
2. Change two links in the default_admin.info.yml to point to the correct documentation pages

Creating this as a bug, as linking to the incorrect documentation is a bug.

Steps to reproduce

Open default_admin.info.yml and see two links to old (outdated) Gin documentation, which does not apply fully to the Default admin theme.

Proposed resolution

Copy the relevant docs, update images.
default_admin.info.yml was updated in #3582169: Cleanup cspell ignores in Admin

Any followups can be done in #3618215: Documentation for Admin theme

Remaining tasks

1. ✅ Create 'Default Admin' in Core themes, https://www.drupal.org/docs/core-modules-and-themes/core-themes
2. ✅ Copy the page about the theme settings.
3. ✅ Update images, after info.yml is updated

default_admin.info.yml was updated in #3582169: Cleanup cspell ignores in Admin

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet