Boost: Better links for feature descriptions

Originally filed by @bobmatyas

On the screen where Jetpack Boost is enabled, it would be nice if we link to better resources than web.dev.


The resources on web.dev are great if you have some knowledge of web performance, but it seems like for the type of audience Boost is aimed at we could do better by presenting a clearer explanation.

#### Describe the solution you'd like

I'd like to see a page on Jetpack.com that explains the Critical CSS, Defer Non-Essential JavaScript, and Lazy Image Loading in plain language that explains what the features do.

We could also note that Lazy Loading adds backwards compatibility to the Lazy Loading in core + doesn't duplicate Jetpack's existing Lazy Loading.

This would have the advantage of being easier to understand and getting folks on Jetpack.com where they may find other performance-related improvements from Jetpack.

This could live as a sub-page with anchor links under:

https://jetpack.com/boost/

Or on the support page:

https://jetpack.com/support/performance/jetpack-boost/