Drop (most of) index, use xref instead

[marcoscaceres]

ReSpec support automatically cross linking to other specs via its xref feature. For specs that are indexed, you can just link to things directly.

I'll send a short pull request to get you started and show you how it's used.

For more guidance, see: https://respec.org/docs/#xref

And: https://respec.org/docs/#Shorthands-Guide

[marcoscaceres]

Remember, you can look up terms/concepts/IDL at https://respec.org/xref/ and it shows you how to cite things.