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>

templates/org/home.tmpl

@@ -11,7 +11,7 @@
 				</span>
 				{{if .IsOrganizationOwner}}<a class="middle text grey" href="{{.OrgLink}}/settings">{{svg "octicon-gear" 16 "mb-3"}}</a>{{end}}
 			</div>
-			{{if $.RenderedDescription}}<p class="render-content markdown">{{$.RenderedDescription|Str2html}}</p>{{end}}
+			{{if $.RenderedDescription}}<p class="render-content markup">{{$.RenderedDescription|Str2html}}</p>{{end}}
 			<div class="text grey meta">
 				{{if .Org.Location}}<div class="item">{{svg "octicon-location"}} <span>{{.Org.Location}}</span></div>{{end}}
 				{{if .Org.Website}}<div class="item">{{svg "octicon-link"}} <a target="_blank" rel="noopener noreferrer" href="{{.Org.Website}}">{{.Org.Website}}</a></div>{{end}}