Add Index of Examples in the Appendix

[MatsPalmgren]

Most CSS specs have a bunch of Examples embedded in the document. It would be really useful to me if there was a generated index in the appendix with links to them to make it easy to access them directly.

CC @tabatkins

[fantasai]

@MatsPalmgren Mind refiling this against Bikeshed? https://github.com/tabatkins/bikeshed/issues

[MatsPalmgren]

Oops, sorry I didn't know there's a separate repo for that.
And there's already an open issue for it: speced/bikeshed#1149