docs: Add subheadings to signals documentation page

Splits the text up a bit and allows linking directly to those subsections.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

docs/reference/gobject/signals.md

@@ -14,6 +14,8 @@ through strings. Signals introduced for a parent type are available
 in derived types as well, so basically they are a per-type facility
 that is inherited.
 
+## Handlers
+
 A signal emission mainly involves invocation of a certain set of
 callbacks in precisely defined manner. There are two main categories
 of such callbacks, per-object ones and user provided ones.
@@ -29,6 +31,8 @@ frequently happens at the end of an object class' creation), while user
 provided handlers are frequently connected and disconnected to/from a
 certain signal on certain object instances.
 
+## Emissions
+
 A signal emission consists of five stages, unless prematurely stopped:
 
 1. Invocation of the object method handler for `G_SIGNAL_RUN_FIRST` signals