Use a generic markup class to display externally rendered files and diffs (#15735)

Use a generic markup class to display externally rendered files and diffs (#15735)

* creates and implements generic markup less class

* How to give custom CSS to externally rendered html

* Clarifies sources of CSS styling of markup

* further clarification of sources of markup styling

* rename _markdown to _markup

* remove defunct import

* fix orphaned reference

* Update docs/content/doc/advanced/external-renderers.en-us.md

* more renames markdown -> markup

* do not suggest less customization

* add back tokens

* fix class whitespace, remove useless if-clause

* remove unused csv-data rules

* use named exports and rename functions

* sort imports

Co-authored-by: HarvsG <11440490+HarvsG@users.noreply.github.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: silverwind <me@silverwind.io>