gh-86726: Document the full public API of tkinter (GH-151579)

gh-86726: Document the full public API of tkinter (GH-151579)

Replace the previously sparse reference documentation with full coverage of
the public API of the tkinter package, written from the Tcl/Tk manual pages,
the existing documentation and the module docstrings.

* Doc/library/tkinter.rst gains a "Reference" section documenting every public
  class, method, function and constant of the core module -- the widgets, the
  Misc, Wm, Pack, Place, Grid, XView and YView mix-ins, the Variable and image
  classes, the module-level functions and the symbolic constants.
* Doc/library/tkinter.ttk.rst, dialog.rst, tkinter.font.rst and the other
  module pages document their remaining classes, methods and functions.

The descriptions are Python-oriented (correct return types -- tuples rather
than Tcl lists, booleans, integers, None on cancellation, and so on) and were
checked against the Tcl/Tk 9.1 manual pages and the implementation.

versionadded, versionchanged and deprecated directives are added for the
public API, determined from the git history relative to Python 3.0: the
tkinter.ttk module (3.1); the Text, Wm, Menu and Misc methods exposing Tk 8.5
features (3.3); and the many later additions and behavior changes up to 3.15.
The Tk version required by features added after Tk 8.6 is noted as well.  The
bundled Tcl/Tk version is updated to 9.0 and the manual-page links point at
the tcl9.0 reference.

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>