docs: Rework READMEs

[kdmccormick]

WIP

Preview: https://github.com/openedx/sample-plugin/blob/kdmccormick/readmes/README.md

Written by Claude from this prompt:

---

Hi, I'd like to rework all READMEs (excluding the one in docs/decisions).

General principles:

• Prioritize the ability of someone jumping into the readme first the first time to (i) understsand the purpose and (ii) find instructions of the common two use cases (local dev with Tutor, local dev without Tutor). Details are de-emphasized by being moved lower nested in more specific READMEs.
• One source-of-truth for any set of instructions. When helpful, link to the source of truth rather than repeating it.
• Avoid "AI-isms" (sorry, Claude :), such as headings like "Key X" and phrases like "The most important thing: ...".
• Avoid sprawling lists of bullet points. We don't need a link to every resource!

Here's my proposed structure--loosely, these are ideas, not headings. Feel free to suggest alternatives and discuss with me.

• ./README.rst
  • What is it?
  • Table linking to sub-plugins
  • Development with Tutor
  • Development without Tutor
  • Getting help
• <sub-plugin>/README.rst
  • What is it?
  • How to use it -> just link back up to top level README
  • How does it work?

Other guidance:

• Preserve existing TODOs
• Ask for questions eagerly
• Don't make assumptions about factual content. Happy to explain any technical information that's missing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.04%. Comparing base (e7a17d1) to head (b51b811).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #52 +/- ##
=======================================
Coverage 93.04% 93.04% =======================================
Files 16 16 Lines 503 503 Branches 18 18 =======================================
Hits 468 468 Misses 24 24 Partials 11 11 

Flag	Coverage Δ
unittests	93.04% <ø> (ø)

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