Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

* Rework 'make generate-images'

- Remove external dependencies and replace it with a node script that
  does does the same.
- Move detail removal from gitea-sm.png to favicon.png
- Remove favicon.ico and its generation, it is unused and we already serve
  favicon.png in its place.

Fixes: https://github.com/go-gitea/gitea/issues/12314

* use proper centering value for preserveAspectRatio

* fix lint

* use fabric

* better linting fix

* fix typo

* mention detail-remove class in docs