[Docs Rewrite] Re-add "Middleware" page in "Understanding Redux"

[markerikson]

---

name: 📖 New/Updated Documentation Content
about: Adding a new docs page, or updating content in an existing docs page

PR Type

New-ish, sorta! Same old "Middleware" page we just removed, but in a new location.

Checklist

• Is there an existing issue for this PR?
  • [Docs Rewrite] Meta-Issue: Tutorials #3595
• Have the files been linted and formatted?

What docs page is being added or updated?

• Section: "Understanding Redux"
• Page: "Middleware"

[github-actions]

Size Change: 0 B

Total Size: 8.9 kB

ℹ️ View Unchanged

Filename	Size	Change
dist/redux.js	6.37 kB	0 B
dist/redux.min.js	2.53 kB	0 B

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