Revert video embed to thumbnail-link, remove redundant endpoint table

[davidchatting-bot]

Follow-up to #78 (merged) - these two commits were pushed to the same branch just after that PR merged, so they need their own PR, same as #78 was itself a follow-up to #76.

• Reverts the `<iframe>` embed back to the clickable-thumbnail version - confirmed GitHub does strip `<iframe>` from rendered READMEs, so the iframe from Embed demo video as iframe, link it from P5js, document endpoint JSON shapes #78 isn't actually displaying on github.com. The thumbnail-link still links to the same video, it's just the version that actually renders.
• Removes the `## Endpoints` summary table - it duplicated the method/path/description already covered by each `### METHOD /path` subsection directly below it, so it was redundant rather than adding anything the subsections didn't already cover.

README only, no code changes.