[3.15] gh-153550: Modernize the older prose sections of the tkinter documentation...

[3.15] gh-153550: Modernize the older prose sections of the tkinter documentation (GH-153647) (GH-153773)

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
  covering grid, pack and place, and cross-link it with the Grid, Pack
  and Place reference classes.
* Rework "The window manager" and "Coupling widget variables", replacing
  the dated App(Frame) examples with direct Tk() and ttk examples.
* Replace page-number citations to Ousterhout's book with links to the
  relevant Tk man pages.
* Modernize the tkinter.ttk front matter and its Tcl/Tk links, and add
  cross-references between the two documents.
* Reconcile the "Packer options" and "Tk option data types" lists with
  the reference, and fill in the empty Entry index list.
* Fix the malformed target-selection list in tkinter.dnd.
(cherry picked from commit 93beea7e5a3c5a03253e6812a06f2497505aeb30)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>