Add UCAN section color coding

[bgins]

A UCAN includes header, payload, and signature sections. Let's color code sections to show how they correspond in encoded and decoded UCANs.

For example, the text of the encoded header and decoded header might be blue. Payloads might be red. We'll need to make some color choices, and it would be a good idea to look at the Carbon Design Color Guidelines.

Note that encoded sections are delimited by a .. Decoded sections are in CodeSnippet components.