Template defs: Hatnotes

[zverok]

https://en.wikipedia.org/wiki/Wikipedia:Hatnote -- it's links (typically at beginning of some section) with text like "See also" or "Main article on this topic is...".

Ideally, there should be:

• Section#hatnote (with aliases like main_article, see_also and so on) and their predicate variants (Section#has_hatnote?)
• Hatnote can be followed, as any normal wikilink