amp-font docs should be more use case driven

[cramforce]

Right now it is super hard to understand what amp-font is for.

@sriramkrish85 Could you work with @Meggin on this? Thanks!

[camelburrito]

Yes i will work with @Meggin . Assigning to her so that she can grab me when she has time.

@cramforce - Can we close this one as this appears to be documented well on https://www.ampproject.org/docs/reference/components/amp-font?