Minor cleanup needed.

Page URL

https://docs.flutter.dev/platform-integration/web/renderers/

Page source

https://github.com/flutter/website/tree/main/src/content/platform-integration/web/renderers.md

Describe the problem

• double title
• remove by-line
• fix &emdash; bits
• general cleanup
• add to index

Expected fix

No response

Additional context

No response

I would like to fix this problem.

• I will try and fix this problem on docs.flutter.dev.