bikeshed and merged docs

Hi there,

I converted the remaining two docs (signature-format and key-attestation)
to bikeshed format, and then merged (basically concatenated) the three docs
into one.

It's in a branch 'doc-merge' (https://github.com/w3c/webauthn/tree/doc-merge),
and it works like this:

- index.src.html: the combined bikeshed source for all three docs
- biblio.json: the bibliography
- Makefile: the Makefile that shows how to invoke bikeshed

- index.html: the bikeshed output.

I haven't removed the three sources in webauth-key-attestation,
webauthn-signature-format, and webauthn-web-api yet (even in that branch),
so people can check out the branch, run diffs etc., to see what I did.

bikeshed (at least the version I have) runs without warnings/errors on
this, but that doesn't mean that currently all the cross-references are
well-formed. Let me know if this looks good to people. If so, I'll nuke the
three old subdirectories (webauth-key-attestation,
webauthn-signature-format, and webauthn-web-api) and pull this into master,
at which point the hard work of polishing this can begin.

Dirk.