docs: clarify Base64 image uploads

[SunPeiYang996]

Summary

Clarify that Markdown image sources do not currently accept Base64 Data URIs, and direct users to upload the image to an HTTP(S) URL first.

Changes

• Document the unsupported Base64 Data URI form in lark-doc-md.md.

Test Plan

• Documentation-only change; reviewed the rendered Markdown and diff.
• Manual local verification confirms the lark-cli <domain> <command> flow works as expected.

Related Issues

• None

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