gh-153550: Modernize the older prose sections of the tkinter documentation

Refresh the narrative "Tkinter life preserver" and "Handy reference"
sections to match the overhauled reference (gh-86726 and gh-153549).

* Rework "The packer" into a general "Geometry management" section that
  covers grid, pack and place, with a short example and use case for
  each, presents grid as the flexible default, and warns against mixing
  pack and grid in one container.  Add symmetric cross-links between the
  Grid, Pack and Place reference classes and this section.
* Rework "The window manager": explain what the window manager is, state
  that the Wm mixin is inherited by Tk and Toplevel so its methods are
  called directly on a top-level window, and replace the dated
  App(Frame) example (which reached the window through master) with a
  direct Tk() example.  Drop the private _root() aside and cross-link
  the section and the Wm reference class to each other.
* Rework "Coupling widget variables": explain that a widget links to a
  Variable object rather than a plain Python variable because Python
  cannot notify Tk when a variable is reassigned, and replace the dated
  App(Frame) example (whose entry was attached to the wrong parent) with
  a direct ttk example showing both get() and set().
* Remove the page-number citations to Ousterhout's book and point to the
  relevant Tk man pages instead.
* Refresh the Tcl/Tk links in tkinter.ttk: add the official ttk::intro
  man page as the current explanation of the theme engine, keep Joe
  English's illustrated 2004 "Tile Widget Set" paper for its element and
  layout diagrams, and drop the now-redundant conversion monograph.
* Modernize the tkinter.ttk front matter: lead the introduction with the
  native-theme benefit instead of peripheral features, update the TIP 48
  link to its canonical URL, and add a note recommending the explicit
  "from tkinter import ttk" form over the star-import override.
* Add cross-references between the two documents: point the tkinter.ttk
  introduction at the shared concepts documented for tkinter, and link
  the themed-widgets note in the tkinter introduction to tkinter.ttk.
* Reconcile the "Packer options" and "Tk option data types" lists with
  the reference: replace the incomplete packer list with a cross-link to
  Pack.pack_configure, and frame the data-types list as the shared value
  types the reference refers to.
* Fill in the empty Entry index list under "The index parameter", and
  describe "@6" as a string rather than an integer.
* Point the classic base-widget examples at the themed ttk widgets.
* Fix the malformed target-selection list in tkinter.dnd: make it a
  single ordered list with the sub-steps nested under the search step,
  and mark up "source" and "event" as parameters.

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

Author: serhiy-storchaka

Doc/library/tkinter.dnd.rst

@@ -16,21 +16,22 @@ a single application, within the same window or between windows. To enable an
 object to be dragged, you must create an event binding for it that starts the
 drag-and-drop process. Typically, you bind a ButtonPress event to a callback
 function that you write (see :ref:`Bindings-and-Events`). The function should
-call :func:`dnd_start`, where 'source' is the object to be dragged, and 'event'
+call :func:`dnd_start`, where *source* is the object to be dragged, and *event*
 is the event that invoked the call (the argument to your callback function).
 
 Selection of a target object occurs as follows:
 
-#. Top-down search of area under mouse for target widget
+#. Top-down search of the area under the mouse for a target widget:
 
- * Target widget should have a callable *dnd_accept* attribute
- * If *dnd_accept* is not present or returns ``None``, search moves to parent widget
- * If no target widget is found, then the target object is ``None``
+   * the target widget should have a callable *dnd_accept* attribute;
+   * if *dnd_accept* is not present or returns ``None``,
+     the search moves to the parent widget;
+   * if no target widget is found, the target object is ``None``.
 
-2. Call to *<old_target>.dnd_leave(source, event)*
-#. Call to *<new_target>.dnd_enter(source, event)*
-#. Call to *<target>.dnd_commit(source, event)* to notify of drop
-#. Call to *<source>.dnd_end(target, event)* to signal end of drag-and-drop
+#. Call to ``<old_target>.dnd_leave(source, event)``.
+#. Call to ``<new_target>.dnd_enter(source, event)``.
+#. Call to ``<target>.dnd_commit(source, event)`` to notify of the drop.
+#. Call to ``<source>.dnd_end(target, event)`` to signal the end of drag-and-drop.
 
 
 .. class:: DndHandler(source, event)