chore(storage-browser): add multi-file download docs examples

[tiffanynwyeung]

Description of changes

• add doc examples for multi-file download
• replace URL result value from new URL('') with null in default download action used in interactive examples
  • Using constructor with invalid empty string throws TypeError which blocks promise from resolving, preventing mock download interactions from rendering properly

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• [x] Have read the Pull Request Guidelines
• [x] PR description included
• [x] yarn test passes and tests are updated/added
• [x] PR title and commit messages follow conventional commit syntax
• [x] If this change should result in a version bump, changeset added (This can be done after creating the PR.) This does not apply to changes made to docs, e2e, examples, or other private packages.

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[changeset-bot[bot]]

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Latest commit: 6551f1d1fea294dd7abe932e2362d27ae3724999

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