docs/develop: Fix some internal links in package documentation.

Use references as documented in https://www.sphinx-doc.org/en/master/usage/referencing.html#ref-role

Fixes #19509

docs/develop/packages/FileFormat.rst

@@ -1,3 +1,5 @@
+.. _hpkg_file_format:
+
 =========================
 Haiku Package File Format
 =========================
@@ -929,6 +931,8 @@ B_HPKG_ATTRIBUTE_ID_PACKAGE_PRE_UNINSTALL_SCRIPT ("package:pre-uninstall-script"
     "boot/pre-uninstall/".
   :Child Attributes: none
 
+.. _hpkr_file_format:
+
 Haiku Package Repository Format
 ===============================
 Very similar to the package format, there's a Haiku Package Repository (HPKR)