📖 Consolidate Metal3 Contributing guide in community repo

[kashifest]

This is the consolidation of Contributing guide where generic contributing guidelines are maintained in the community repo and individual repos only contain repo-specific information.

Fixes #

Checklist:

• Documentation has been updated, if necessary.
• Unit tests have been added, if necessary.
• E2E tests have been added, if necessary.
• Integration tests have been added, if necessary.

[metal3-io-bot]

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[kashifest]

/hold
Reviews are welcome but this should land after metal3-io/community#40 lands

[tuminoid]

/cc @tuminoid

[tuminoid]

Pretty much same things mentioned as in CAPM3.

AGENTS.md reference for "technical contribution guide" or something needed.

[metal3-io-bot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale.
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/lifecycle stale

[tuminoid]

/remove-lifecycle stale

@kashifest we need to follow-up on these repo specific updates.

[kashifest]

@tuminoid @lentzi90 PTAL again

[kashifest]

/hold cancel

[lentzi90]

/lgtm

[Copilot]

Pull request overview

Consolidates this repository’s contributing documentation by linking to the shared Metal3 community contributing guide and keeping only Baremetal Operator–specific policies locally.

Changes:

• Replaces most repository-specific contributing text with links to the common Metal3 community CONTRIBUTING guide sections (versioning, branches, backporting, release process).
• Keeps Baremetal Operator–specific maintenance/backporting notes and points release steps to docs/releasing.md.

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[Copilot]

The note mentions "experiments" not providing backward compatibility guarantees, but this repository doesn't appear to have an experiments module/directory. This makes the guidance confusing/outdated; consider removing the "experiments" reference or pointing to the actual location/name used in this repo (if any).

Suggested change

	**Note**: The test module and experiments do not provide any backward
	compatible guarantees.
	**Note**: The test module does not provide any backward compatible
	guarantees.

[Copilot]

PR description includes an unfinished "Fixes #" reference. Please either link the issue being fixed (e.g. "Fixes #123") so it auto-closes on merge, or remove the placeholder if there's no associated issue.