gh-153550: Keep the old the-packer and packer-options anchors

Renaming the "The packer" section to "Geometry management" and folding
away the "Packer options" subsection removed their implicit HTML ids,
breaking existing deep links.  Restore both as explicit targets on the
new section.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Doc/library/tkinter.rst

@@ -586,6 +586,8 @@ their values.  This is meant only as an example.
 
 .. _pack-the-packer:
 .. _tkinter-geometry-management:
+.. _the-packer:
+.. _packer-options:
 
 Geometry management
 ^^^^^^^^^^^^^^^^^^^