Add performance relayer + documentation (web-vitals)

[housseindjirdeh]

Fixes#8996

Changes included:

• Dynamically imports the web-vitals lib
• Includes a reportWebVitals function to index.js template (doesn't do anything until a function is passed in)
• Adds a "Measuring Performance" page to the documentation

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[mrmckeb]

I haven't tested this, but it looks good at an overview, and the docs look great. This is something we definitely talked about getting into the next release - and I'd love to see it make it.

Thoughts @ianschmitz@iansu?

[kentcdodds]

FWIW, I think this is good to go 👍

[mrmckeb]

I've tested this on a project today and it works well. Obviously the extension works with or without this, but being able to report on this is really valuable.

[ianschmitz]

Thanks a ton @housseindjirdeh! Much appreciated.

[housseindjirdeh]

Awesome thanks all! So happy to see this land :)