RFC 9637 expands the IPv6 Documentation Space by 3fff::/20

[aaaaalbert]

RFC 9637 was just released, and it describes a new reserved IPv6 address prefix for documentation purposes, namely 3fff::/20. The reasoning is that the existing documentation prefix, 2001:db8::/32, is too small to reflect "many realistic, current deployment scenarios."

Adding this new doc prefix to CNP3 is either very simple (as in: add one line to mention the additional block and its RFC), or very time-consuming (as in: make good use of the new prefix in new examples throughout, etc.)

I can probably offer to implement the first option if you don't mind to wait a little while, maybe one month or so, for a pull request.