docs: restore README build matrix

[zackees]

Summary

• restores the full per-board build matrix to the README front door
• keeps the matrix expanded by default with no <details> wrapper or click-to-expand step
• groups badges by platform family and links deeper board notes to docs/BOARD_STATUS.md

Checks

• git diff --check
• README workflow badge audit: every actions/workflows/*.yml badge target exists
• RUSTDOCFLAGS="-D warnings" soldr cargo doc --workspace --no-deps

Summary by CodeRabbit

• Documentation
  • Added a new "Build Matrix" section to the README detailing CI coverage across supported board families (AVR, ESP8266/ESP32, RISC-V, Teensy, STM32, RP2040/RP2350, Nordic NRF52, and others).
  • Included GitHub Actions badges for each board workflow.
  • Added reference link to board status documentation for detailed per-family information.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 3dc3086c-afec-426e-9872-8eda50ab5a5b

📥 Commits

Reviewing files that changed from the base of the PR and between 4ca6990 and f3229d5.

📒 Files selected for processing (1)

• README.md

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📝 Walkthrough

Walkthrough

This pull request adds a new Build Matrix section to the README that documents CI coverage across multiple board families. The section enumerates supported boards with GitHub Actions badge links and directs readers to docs/BOARD_STATUS.md for comprehensive board family details and status information.

Changes

Build Matrix Documentation

Layer / File(s)	Summary
Build Matrix section with board families and badges README.md	New "Build Matrix" section inserted after badges and before Installation, listing supported board families (AVR, MegaAVR, Renesas, ESP8266/ESP32, RISC-V variants, Teensy, STM32, SAM/SAMD, RP2040/RP2350, Nordic NRF52, Apollo3, NXP LPC, Silicon Labs, Raspberry Pi Pico) with CI workflow badge links and reference to docs/BOARD_STATUS.md.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Possibly related PRs

• FastLED/fbuild#82: Earlier PR that removed the Build Matrix from README in favor of docs/BOARD_STATUS.md; this PR reintroduces the matrix as a summary with badge links back to the documentation.

Poem

🐰 The matrix blooms upon the page,
With boards in badges, all the rage,
From AVR's wings to RISC-V's flight,
CI workflows glow, a checkered sight!
Status deeper in the docs we'll find,
A roadmap for the maker's mind. 🚀

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The PR title 'docs: restore README build matrix' accurately summarizes the main change: restoring a build matrix section to the README documentation.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/readme-build-matrix-front

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