[vpdq] Add data flow diagram, misc docs cleanup

[ianwal]

Summary

I was looking at vPDQ again and noticed the initial docs has a lot of words and can be hard to understand.

Tabulating (?) the TMK comparison and adding a simple high level diagram of how vPDQ works can help new readers understand the project better.

• Add simple high level mermaid diagram for vPDQ data flow.
  • github supports rendering mermaid diagrams. VSCode can render markdown, but currently requires an extension like Markdown Preview Mermaid Support to render it.
  • This diagram doesn't really follow any particular format like UML.
• Put TMK+PDQF / vPDQ comparison in table form for readability
• Other misc cleanup
  • All changes maintain the same amount of info but in a different format that may be easier for new users to understand.

Consideration: mermaid can also be exported to svg, so instead of a codeblock I could add a svg file to avoid readers needing a mermaid renderer. Probably will still need to put the mermaid code somewhere since I think mermaid -> svg is a one way conversion.

Test Plan

Mermaid renders as expected in github: