Make README clearer and more concise

[fern-api]

Summary

Trim and reorganize the repo README so contributors can scan it quickly.

What changed

• Replaced the wordy intro with a one-line description of what this repo is.
• Collapsed the two-step ''Installation'' and three-step ''Editing documentation'' flows into a single, tighter ''Run the docs locally'' quickstart.
• Simplified the prerequisites bullet (single line covers both Node.js and npm).
• Tightened the writing-tips bullets without dropping any guidance.
• Condensed the Vale intro and the collapsed style-guideline lists.
• Reworded the review-process section into two clear bullets.
• Replaced time-relative phrasing (''currently live'', ''Finally, when you make a PR'') per the in-repo style rules.

No links, code blocks, or substantive guidance were removed — every section in the original README is still present, just shorter.

Requested by: Fern Support

[github-actions]

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