The example macro from the Help application should be internal and fixed

The example macro shipped by the Help application (Help.Code.ExampleMacro) is only meant to build the Help pages: it is used 19 times across the Help.Macros.* pages to show a rendered example next to its source. It is not documented anywhere for end users, yet it is exposed to them like any other macro, which adds noise when inserting a macro. See the analysis in https://design.xwiki.org/xwiki/bin/view/Proposal/WYSIWYGusabilityofXWikimacros#Hexample

Problems

• The macro default category is Formatting, so it is listed in the WYSIWYG macro picker and in the quick actions next to user-facing macros.
• When inserted from the WYSIWYG editor, its content cannot be edited, even though the content is mandatory and is the only input of the macro.
• The "Example" and "Source" tab labels are hardcoded in English instead of using translation keys.
• The tab ids are computed from the content hash plus a random number between 1 and 100, so two examples on the same page can end up with the same id and break tab switching.
• The border color of the tab content is hardcoded to #DDD instead of following the color theme.

Proposal

• Move the macro to the Internal category so that it is hidden by default from the macro picker. Hiding it from the quick actions as well depends on XWIKI-21063.
• Translate the tab labels.
• Generate unique tab ids.
• Use a color theme variable for the border.