bpo-42560: rewrite of Tkinter docs "life preserver"

[roseman]

This PR is effectively a complete rewrite and reorganization of the existing 'Life Preserver' section, and as such it's more useful to compare them in their entirety rather than trying to parse line-by-line diffs. Realistically, I don't think it would be useful to try to break this up into smaller PR's.

The goals, as stated in the new intro:

• quick orientation to what Tkinter app looks like
• identify foundational concepts
• explain structure of Tkinter wrapper
• how to identify classes, methods, etc. you'll need
• how to find detailed documentation elsewhere

To summarize the changes:

• drop credits section

• replace specific reference to man page sections, old Ousterhout book, Tkinter source code with pointer back to previous list of external references

• hello world:

  • replaced class-based (so much shorter, more direct)
  • now uses ttk and grid
  • added line-by-line walkthrough

• important tk(inter) concepts

  • new section
  • some of this was muddled in the previous Tcl-oriented sections, but most not there

• understanding how tkinter wraps tcl/tk

  • made shorter, more descriptive, less indirect/illustrative explanation of concepts
  • less walking through actual Tcl syntax
  • emphasizes important points of difference

• how do I ... / what option...

  • this was previously dispersed across several sections, now more task-oriented
  • assumption is that external documentation will be used, and knowing the right term to search for is key
  • remind about introspection facilities for methods and objects, and also identifying those unique to a particular widget

• navigating tcl/tk reference manual

  • solves the problem of given name of method, where to go find it in man pages?
  • explains that while in tkinter everything is a method, in Tcl/Tk some are widget-specific methods, others are toplevel commands (e.g. grid)
  • points to pages for common options and methods
  • describes where to find docs on methods with compound names, e.g. "winfo_..."
  • notes gotcha that some things that look like methods for a specific widget actually don't operate on the widget

https://bugs.python.org/issue42560

[miss-islington]

Thanks @roseman for the PR, and @ambv for merging it 🌮🎉.. I'm working now to backport this PR to: 3.9, 3.10.
🐍🍒⛏🤖

[miss-islington]

Sorry @roseman and @ambv, I had trouble checking out the 3.10 backport branch.
Please backport using cherry_picker on command line.
cherry_picker 08830a4f75553682e8d1ea6cdabd4c3472c59e28 3.10

[bedevere-bot]

GH-27911 is a backport of this pull request to the 3.9 branch.

[miss-islington]

Thanks @roseman for the PR, and @ambv for merging it 🌮🎉.. I'm working now to backport this PR to: 3.9.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

[miss-islington]

Thanks @roseman for the PR, and @ambv for merging it 🌮🎉.. I'm working now to backport this PR to: 3.10.
🐍🍒⛏🤖

[bedevere-bot]

GH-27912 is a backport of this pull request to the 3.10 branch.