feat(multimodalparser): add design doc

[bridgeqiqi]

Description

Refer to the #59

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with $CATEGORY(xx): xxx (such as feat(tool): xxx, fix(source): xxx, docs(README): xxx)
• Related issue is referred in this PR
• The markdown and latex are rendered correctly.
• The code in PR is well-documented.
• The PR is complete and small, read the Google eng practice (CL equals to PR) to understand more about small PR.

[keli-wen]

trim trailing whitespace.................................................Failed
- hook id: trailing-whitespace
- exit code: 1
- files were modified by this hook

Fixing docs/design/en/multimodal_content.md

Please first fix it and upload the design docs in the notion? (This makes it easier to review.)