[R2] Update custom domain docs to align with new UI

[smellercf]

Summary

Updates the R2 public bucket documentation to align with the recently released UI for managing public access settings. It also adds guidance on how to serve public R2 content using non-Cloudflare-managed domains.

Screenshots (optional)

Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

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PR Change Summary

Updated custom domain documentation to reflect changes in the user interface and improve clarity.

• Revised terminology from 'Incremental migration' to 'On Demand Migration' in migration documentation.
• Clarified steps for connecting a custom domain to an R2 bucket.
• Updated cautionary notes regarding public access to R2 buckets.
• Enhanced instructions for enabling and disabling public development URLs.

Modified Files

• src/content/docs/r2/buckets/public-buckets.mdx
• src/content/docs/r2/data-migration/sippy.mdx
• src/content/partials/r2/custom-domain-steps.mdx

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[Oxyjun]

Connect a bucket to a custom domain

Is this a reference to a component on the dashboard? I wasn't 100% sure which page I needed to be looking at here

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