bpo-42560: rework external references in Tkinter docs (GH-27838)

- reorganized from two sections (tkinter vs. tcl/tk) into three (tkinter, tcl/tk, and books)
- main (recommended) tkinter docs have one tutorial (tkdocs) and one reference (shipman), added better descriptions
- dropped link to Tkinter page on wiki (suggestion by E. Paine; outdated, most material already linked to from python.org)
- replaced Tcl/Tk recent man pages and core dev home with single link to main Tcl/Tk page (which holds both of these)
- updated Modern Tkinter link to book page on TkDocs site (was Amazon link to old version), dropped description
- replaced Grayson book by Moore book (newer, covers ttk)
- changed Ousterhout ref to second edition, covers ttk
- dropped link to Welch book (old)