Add extension.json and populate it with information from both
RelatedArticles.php and the RelatedArticles extension wiki page [0].
Migrate the hook handlers to static functions operating on static data
rather than having a single, global instance of RelatedArticles so that
the hook handler definitions can be added to extension.json easily.
N.B. During testing the handlers for both the SkinBuildSidebar and
SkinTemplateToolboxEnd appeared to have the same effect so the TODO was
removed but not the handler for the former.
Additional changes include:
* RelatedArticles.class.php -> includes/RelatedArticles.php
* RelatedArticles -> RelatedArticles\Hooks
* Documenting all of the methods of RelatedArticles\Hooks
[0] https://www.mediawiki.org/wiki/Extension:RelatedArticles
Bug: T87965
Change-Id: I9944b9186746386ee18ca28657bb547c00ae2b8c
