Add documentation column in the syntax guide for macros

I'm not sure I agree with the proposal (which should be a real proposal on the forum ideally):

• The syntax guide you mention is to explain the xwiki syntax to call a macro. It's not meant to explain how to use macros.
• The help for macros inside XS is at http://localhost:8080/xwiki/bin/view/Help/Macros/ and it has a link to list all existing macro docs at https://extensions.xwiki.org/xwiki/bin/view/Main/#|t=extensions&p=1&l=30&s=doc.creationDate&d=desc&category=macro (the LT is not filtered on macros for me, we need to fix this). This is where the user is supposed to find the full doc on each macro.
• There's a macro list inside XS listing all macros, linked from the Macro help from http://localhost:8080/xwiki/bin/view/Help/Macros -> http://localhost:8080/xwiki/bin/view/XWiki/XWikiSyntaxMacrosList . When the macros are wiki macros, there's a link to the home page of the macro which could list some example usage of the macro.

Note that we could put a link to each macro specifically from inside XS but we would need to maintain it, so an option would to have a generic page on xwiki.org (as we have on exo's Extension space) to redirect to the macro doc in a generic manner, using the macro's extension id.