docs: Update mypy documentation on django-stubs

[PIG208]

This is a follow-up to #18777.

Some additional documentation is added regarding features supported by django-stubs.

Screenshots and screen captures:

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[timabbott]

I did a bit of copyediting, mainly to describe what a QuerySet is with an example before we talk about those in detail, and then merged this. Thanks @PIG208! I'd appreciate your advertising these documentation updates and asking for feedback; it's possible even with my edits we don't provide enough context.