docs: adding documentation on how to do initial publish

[joker23]

This PR adds the initial documentation on how to publish a package in the new npm OIDC world. This is the first steps to formalize this process and I plan to provide a more scripted process which will come along with the next new package.

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Note

Introduces a comprehensive initial publishing guide in contributing/publishing.md for new SDK packages.

• Adds Step 0 to create a placeholder npm package and configure OIDC Trusted Publishing, including npm publish --tag snapshot
• Details extending release-please-config.json (with release-as, bump-minor-pre-major, and bootstrap-sha tips)
• Adds instructions to update .release-please-manifest.json and manual workflow inputs
• Provides a CI workflow template for per-SDK build/test and notes on local testing
• Clarifies automated vs. manual publishing workflows and options (prereleases, dry-run)

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