Add docs for creating beta releases

[rygine]

Summary

• Updated the contributing guide to outline the steps necessary for creating a beta release

[cloudflare-workers-and-pages]

Deploying xmtp-js-docs with    Cloudflare Pages

Latest commit:	b9a48bb
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Preview URL:	https://8fbb5774.xmtp-js.pages.dev
Branch Preview URL:	https://rygine-beta-release-docs.xmtp-js.pages.dev

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Latest commit: b9a48bb

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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