Add cross references

[martinthomson]

This adds cross references for all uses of the names of frames, transport parameters, and error codes. It should make the document more usable as a result.

A side-effect of this is that there is now an index.

[MikeBishop]

I can't get this to build locally, even though I seem to be on reasonably current versions of both kramdown-rfc2629 and xml2rfc. It fails with a bunch of...

/home/mbishop/github/quic/rfc9000.xml(3139): Error: Did not find an anchor to use for <iref item='PADDING'> in <td>
/home/mbishop/github/quic/rfc9000.xml(3148): Error: Did not find an anchor to use for <iref item='PING'> in <td>
/home/mbishop/github/quic/rfc9000.xml(3157): Error: Did not find an anchor to use for <iref item='ACK'> in <td>
/home/mbishop/github/quic/rfc9000.xml(3166): Error: Did not find an anchor to use for <iref item='RESET_STREAM'> in <td>
/home/mbishop/github/quic/rfc9000.xml(3175): Error: Did not find an anchor to use for <iref item='STOP_SENDING'> in <td>
/home/mbishop/github/quic/rfc9000.xml(3184): Error: Did not find an anchor to use for <iref item='CRYPTO'> in <td>

[martinthomson]

I had no such trouble; I'm running kramdown-rfc 1.6.1 and xml2rfc 3.12.3 (both freshly updated).

[MikeBishop]

I had two copies competing. 3.12.1 was installed, but a copy of 3.5.0 was taking precedence. I removed that and 3.12.1 builds it successfully.