[ENHANCEMENT] Link to MDN docs

**Is your feature request related to a problem? Please describe.**
When I want to see the documentation for one of the features, I have to click through a few webpages to get there (e.g. for https://webstatus.dev/features/font-size-adjust, I can click the "CanIUse" button, and then the Resources tab and then there's a link to the MDN docs), but some pages have no links at all, so I have to search Google or MDN directly for documentation.

**Describe the solution you'd like**
A direct link to the feature's documentation on MDN

**Describe alternatives you've considered**
Manually searching for documentation myself.

**Additional context**
It appears this was an intended feature, but #186 removed it due to lack of data. So I wanted to add this to track adding that capability.