Random walk documentation improvements

[evildmp]

Across the documentation there are numerous examples of scope for improvement. Many of these could be described as housekeeping opportunities - documentation that is fundamentally sound, but needs to be updated and tidied up.

An example is Porting Python 2 Code to Python 3, which was written in a different era.

These are the kind of thing that will be discovered in a random walk through the documentation, one thing leading to another. A random walk will also uncover (often small) possibilities for improvement that would otherwise likely hide unnoticed in quiet corners.

Pitch

I propose taking a random walk through the documentation, finding and taking low-hanging fruit where I go. As well as the value of the fixes themselves, the activity and discussion of issues as they come up with bring further opportunities to the surface, and place them in front of more eyes.

Discoveries and suggestions

• https://docs.python.org/3/howto/annotations.html is not really a how-to - it's an explanation-type document
• https://docs.python.org/3/howto/functional.html should be moved to an explanation section
• https://docs.python.org/3/howto/isolating-extensions.html is explanation

Linked PRs

• gh-107432 Update Porting Python 2 Code to Python 3 how-to #107434
• gh-107432 Fix incorrect indentation in annotations HOWTO #107445
• gh-107432: Rework the style of Doc/howto/functional.rst #107449
• [3.12] gh-107432 Fix incorrect indentation in annotations HOWTO (GH-107445) #107654
• [3.11] gh-107432 Fix incorrect indentation in annotations HOWTO (GH-107445) #107655
• [3.12] gh-107432 Update Porting Python 2 Code to Python 3 how-to (GH-107434) #108409
• [3.11] gh-107432 Update Porting Python 2 Code to Python 3 how-to (GH-107434) #108410