Add new RTK Query pages to the "Essentials" tutorial

[markerikson]

Overview

This PR:

• Adds new pages to the Essentials tutorial covering how to use RTK Query
• Cleans up the Essentials tutorial to reflect updates in usage patterns (such as the "builder callback" syntax for extraReducers)
• Fixes several bits of explanation that had been skipped over during the original tutorial content for space and were identified as being confusing

Status

This is a very rough first draft of the first half of the planned content. As in, I just got done writing for the last few hours :)

So far I've got a "Part 7: RTKQ Basics" page written up, as seen here. I plan to add a "Part 8: RTKQ Advanced Patterns" page covering the rest of the example commits (page name TBD).

I've just added the "Advanced" page first draft to go along with the "Basics" page.

I've also got a third draft of the planned example app code completed, and have just pushed that up to https://github.com/reduxjs/redux-essentials-example-app/tree/tutorial-steps-2021-08-14.

I also will need to go through and do some more cleanup of the earlier sections of the Essentials tutorial, applying some of the feedback from existing issues.

I've got the content basically complete at this point. New pages should be done. Fixed up the existing pages to match the latest code and fill in some missing explanations, and addressed the review comments.

Only thing that should be left at this point is updating the example repo with the final code in the tutorial-steps branch, adding the right tags, and filling in the Sandboxes for the new pages.

The nearly-final example code is viewable at https://github.com/reduxjs/redux-essentials-example-app/tree/tutorial-steps-2021-08-28 .

The new content is previewable at https://deploy-preview-4153--redux-docs.netlify.app/tutorials/essentials/part-7-rtk-query-basics .

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[github-actions]

Size Change: 0 B

Total Size: 8.61 kB

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Filename	Size	Change
dist/redux.js	6.89 kB	0 B
dist/redux.min.js	1.72 kB	0 B

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[phryneas]

Read through part 7, all I have are nitpicks. This is already very polished, like always :)
Probably gonna read through part 8 tomorrow.

[Shrugsy]

Awesome content @markerikson!

I've left some nitpicks/questions/spelling-fixes to look at

[markerikson]

Ready to roll!