Switch library documentation to gi-docgen

Switch from gtk-doc to gi-docgen for the library documentation and
adjust the source files accordingly.

We don't bother about the daemon doc headers as they're not built into
any docs atm.

Closes: #56

doc/libfeedback.toml.in

@@ -0,0 +1,42 @@
+[library]
+version = "@VERSION@"
+description = "A library to trigger feedback (such as haptic feedback) via events sent by your application."
+authors = "Purism SPC"
+license = "LGPL-2.1-or-later"
+browse_url = "https://source.puri.sm/Librem5/feedbackd/"
+repository_url = "https://source.puri.sm/Librem5/feedbackd.git"
+website_url = "https://source.puri.sm/Librem5/feedbackd/"
+dependencies = [
+  "Gio-2.0",
+  "GLib-2.0",
+]
+devhelp = true
+search_index = true
+
+[dependencies."Gio-2.0"]
+name = "GIO"
+description = "GObject Interfaces and Objects, Networking, IPC, and I/O"
+docs_url = "https://docs.gtk.org/gio/"
+
+[dependencies."GLib-2.0"]
+name = "GLib	"
+description = "The base type system library"
+docs_url = "https://docs.gtk.org/glib/"
+
+[theme]
+name = "basic"
+show_index_summary = true
+show_class_hierarchy = true
+
+[source-location]
+# The base URL for the web UI
+base_url = "https://source.puri.sm/Librem5/feedbackd/-/blob/main/"
+# The format for links, using "filename" and "line" for the format
+file_format = "{filename}#L{line}"
+
+[extra]
+content_files = [
+  "overview.md",
+  "build-howto.md",
+  "initialization.md",
+]