[3.15] gh-86726: Improve the structure of the tkinter reference (GH-152109 (GH-152111)

Add short group intros before each cluster of Misc methods.
Group the Tk and Toplevel classes in a new "Toplevel widgets" section, move
the Tcl() function to the module-level functions, and move the "File
handlers" section into the reference.
(cherry picked from commit c7faa69)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka