Add suggestions on how to search the spec?

[EricCousineau-TRI]

Per this issue: gazebosim/sdformat#267

I was trying to find where //surface//friction lived. I briefly looked at places like //link and //world, and tried a few other places, but gave up after 2min.

In the end, I just went to sdformat/sdf/1.7, searched for "surface", found it in surface.sdf, and then searched for that filename to find in included via "collision.sdf".

At a minimum, there should be a suggestion on how to find things.
Ideally, there's an easy way to add simple search functionality, such that you can easily find a tag and its usage context.