docs: simplify README installation guide

[jaibhasin]

Simplifies the README around PRBot's purpose, installation, and review flow.

Keeps a copy-paste GitHub Actions workflow, essential safety notes, and a small optional configuration example.

This makes the project easier to understand and install without losing the key operating details.

Summary by CodeRabbit

• Documentation
  • Simplified and reorganized the README for quicker onboarding.
  • Added a concise three-step installation guide, including API key setup and workflow creation.
  • Condensed the usage overview, optional configuration instructions, and supported commands.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch jaibhasin/concise-readme-install-guide

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🧹 Nitpick comments (1)

README.md (1)

58-59: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Make the optional Action configuration discoverable.

This section names configurable capabilities but provides neither the actual with: input names nor a link to the authoritative input reference. Add a compact example or link to action.yml, and verify the names match the supported inputs.

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@README.md` around lines 58 - 59, Update the README configuration section to
make the optional Action settings discoverable by adding a compact `with:`
example or link to the authoritative `action.yml` input reference. Use the exact
supported input names from `action.yml`, covering the documented model, time,
cost, token, concurrency, and comment-count settings.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@README.md`:
- Around line 58-59: Update the README configuration section to make the
optional Action settings discoverable by adding a compact `with:` example or
link to the authoritative `action.yml` input reference. Use the exact supported
input names from `action.yml`, covering the documented model, time, cost, token,
concurrency, and comment-count settings.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: c3a75645-5510-41c9-89ed-d2f4443b49b9

📥 Commits

Reviewing files that changed from the base of the PR and between 493f7e0 and cf8322e.

📒 Files selected for processing (1)

• README.md