Add accessibility documentation quick start

[musaabhasan]

Summary

• add a quick-start section for writing project accessibility docs
• include a reusable page pattern for product-specific accessibility documentation
• add an alt text decision guide and review checklist for maintainers
• add the split-out lxml_html_clean package required by the current Read the Docs build environment

Refs #15.

Validation

• git diff --check
• python -m pip index versions lxml_html_clean