Provide an entry point for XWiki.WikiMacros

Currently you can access this page only if you know where is located / or by search. It is not a hidden page so this means users should be able to locate it.

There are several remarks regarding this issue:

• we could move this page from XWiki space to Macros space (XWIKI-13498)
• currently Macros.WebHome does not exist. This page content could be ideal for Macros space;
• we don't have any standard that wiki macros should be created in XWiki or in Macros space;
• we have wiki macros definitions in XWiki, Main, Macros spaces and also in the application specific spaces;
• the entry point could be Macros.WebHome marked visible or maybe an Administration section for macros.
• although the wikimacro module is in xwiki-platform-rendering, maybe we could have something like xwiki-platform-macros?

Let me know what you think.