feat: HTML scanner for text content & content sectioning elements → segment paragraphs

[tianjianjiang]

See https://docs.google.com/document/d/1S2gQrOZl5UVwILboc9E4wwmXJtO2QmfNq4B_pTdzYS0
related: #114 blocked_by
related: #124 child

Acceptance Criteria

(TBD)

Given

When

Then