docs: Update private-registries.md to include Cloudsmith support

[himanshuvMishra5]

Adding instructions for Cloudsmith registry support

PR Checklist

• [x] The PR title follows conventional commits
• [ ] Is this closing an open issue? If so, link it, else include a proper description of the changes and rason behind them.
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• [ ] Does the PR have changes to the backend? If so, make sure tests are added.
  And if changing dababase queries, be sure you have ran sqlx prepare and committed the changes in the .sqlx directory.

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