Add build_id reference to tags documentation

[beran-t]

Summary

• Adds a new "Referencing a specific build" subsection under "Use cases" on the tags & versioning page
• Documents the <template>:<build_id> and <namespace>/<template>:<build_id> format for starting sandboxes from a specific build
• Includes Python and JS/TS code examples

Test plan

• Verify the page renders correctly with npx mintlify dev
• Confirm code examples display in both language tabs

[dobrac]

I think this should be under the default tag in the main section