Add documentation page "Writing Custom Middleware"

[dbartholomae]

Fixes #4472

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

PR Type

Add a page

Checklist

• Is there an existing issue for this PR?
  • Share expectations/best practices for Redux middleware #4472
• Have the files been linted and formatted?

What docs page is being added or updated?

• Section: Usage Guide/Using Redux/Redux Logic and Patterns
• Page: Writing Custom Middleware

For Adding New Content

What kind of content category is this page (tutorial, how-to, explanation, reference)?

How-to

Who is the intended target audience?

Developers thinking about writing their own middleware with low to medium experience with Redux.

What knowledge are we assuming they have?

How to use Redux and middleware in general, but not how to write middleware.

What are the intended results or takeaways from reading this page?

• Understanding what middleware can do
• Understanding when to write custom middleware vs. using existing packages
• Understanding how to write middleware that is compatible with other Redux middleware like RTK Query middleware

What is the most critical info they should learn?

• That they most likely shouldn't write custom middleware

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[markerikson]

Great work! Left some requests for tweaks

[dbartholomae]

I've fixed most of these. Open questions are

• how to name the pattern of a middleware modifying its return value
• whether to use the void convention for unawaited promises

[dbartholomae]

@markerikson do you have feedback for the last two questions?

[markerikson]

Per the "naming" question, I think I'd just refer to it as "overriding" or "replacing" the return value.

[dbartholomae]

@markerikson I've rewritten the standard examples with concrete examples from rtk.

[dbartholomae]

@markerikson any feedback?

[markerikson]

Okay, let's go with this. Thanks!