docs: Move floating refs documentation to Markdown

And add some new sections on strategies for avoiding designing APIs
around floating refs.

Helps: #3037

docs/reference/gobject/gobject.toml.in

@@ -42,6 +42,7 @@ show_class_hierarchy = true
 urlmap_file = "urlmap.js"
 # The same order will be used when generating the index
 content_files = [
+  "floating-refs.md",
   "boxed.md",
   "enum-types.md",
 ]