Add links to config var descriptions

[vipulgupta2048]

Redirecting folks to the right context is always valuable. Hence, what if the description property of config variables have links? Here, I mean

https://github.com/balena-io/open-balena-api/blob/7b2e53c0685b65a721e90bb9bc8ac50af39cb818/src/features/vars-schema/env-vars.ts#L157

We can have a link pointing to a place where folks can read more about Dtoverlay. The same could be used by the UI as well?

Idea came from having links in the description fields of this table https://github.com/balena-os/balena-supervisor/blob/master/docs/configurations.md

Context: https://www.flowdock.com/app/rulemotion/r-supervisor/threads/g5SxblwceDmrBTT8ROiR3BQCyIm