Modify docs: Add showcase rules (#3300)

## Review

A dedicated reviewer checked the rule description successfully for:

- [ ] logical errors and incorrect information
- [ ] information gaps and missing content
- [ ] text style and tone
- [ ] PR summary and labels follow [the
guidelines](https://github.com/SonarSource/rspec/#to-modify-an-existing-rule)

---------

Co-authored-by: nicolas-gauthier-sonarsource <121794895+nicolas-gauthier-sonarsource@users.noreply.github.com>

docs/description.adoc

@@ -6,6 +6,11 @@ This document describes how `+rule.adoc+` and its dependencies should be structu
 
 See also the <<styling_guide.adoc#,Styling Guidelines>>, <<tone_guide.adoc#,Tone Guide>> and https://docs.sonarqube.org/latest/extension-guide/adding-coding-rules/#coding-rule-guidelines[Coding rule guidelines].
 
+For technical support, see also
+
+* Rule <<../rules/S6620/java/rule.adoc#,S6620>>, which showcases asciidoc features that are available when writing a rule description.
+* Rule <<../rules/S6778/java/rule.adoc#,S6778>>, which showcases multi-framework rules.
+
 == Sections
 
 There should be no first level titles (`+= Title+`) in your adoc.