Add well-known synchronization points to Feature dependencies

[Enrico Zini]

(from a conversation with @DonKult)

Related: I had hoped the doc would tell me what is a sensible choice as
a dependency. The hello feature is simple enough and the finalize
comment explains the dependencies, but I think that deserves some named
entry points so my "tags-ng" feature can be put before
"_autogenerated_pages" and "hello" after it.
And without the finalize, what would be a good dependency then?

I like the idea of well known synchronization points, like systemd kind of does with .target units. I can think of "contents loaded" and "autogenerated pages" as two useful such points.