Add suggestions on how to search the spec?

Per this issue: https://github.com/osrf/sdformat/issues/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.