docs(capm3): add Unhealthy Annotation section and update SUMMARY

[acheampongmaa]

This PR adds documentation under Cluster API Provider Metal³ (CAPM3) to describe
the capi.metal3.io/unhealthy annotation.

Summary of changes

• Added a new section “Unhealthy Annotation” under capm3/introduction.md.
• Explained how operators can manually mark a BareMetalHost as unhealthy.
• Described the automatic annotation applied when remediation timeouts expire.
• Updated SUMMARY.md to remove a duplicate entry causing mdBook build errors.

Verification

• Verified locally using mdbook serve, the new section renders correctly and links to the Remediation page.

Issue

Fixes Document unhealthy annotation #403

Notes

This is my first contribution to the Metal³ documentation.
Feedback on placement, tone, or level of detail is very welcome.

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[metal3-io-bot]

Hi @acheampongmaa. Thanks for your PR.

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[acheampongmaa]

Hello @dtantsur and @smoshiur1237
This is my first contribution to the project.
I’d really appreciate any feedback or suggestions.
Thank you for your time and guidance!