doc: add a HOWTO for vde_router

[zirize]

Authored by Claude Code (Claude Opus 5). I reviewed it, ran every command in it
on my own hardware, and I am submitting it.

Depends on #74. Without those fixes vde_router does not forward at all,
so a document describing how to route with it would be describing something
that does not happen. This branch is stacked on that one; GitHub therefore
shows its commits here too, and only the last one belongs to this PR. I can
rebase onto master once #74 lands, or fold this in there, whichever you
prefer.

Why

doc/ carries a HOWTO or a README for most of what vde-2 ships — vde_autolink,
vdeqemu, slirpvde, vde_over_ns, vde_vxlan — and nothing at all for
vde_router.

The manual page (rewritten in #74) is reference material: it says what each
option and each management command does. This is the other half — how to get
from nothing to a working router, and what to look at when it does not work.

What is in it

• What it is for, and when a switch alone is enough instead.
• A first router between two switches, with the configuration file and the
  checks that tell you it came up.
• Putting hosts on it — vdens, qemu, or anything else that speaks to a
  switch — and handing out addresses with the built-in DHCP server.
• Reaching the outside. This is where people get stuck: vde_router does no
  address translation, so the uplink has to be an interface connected to
  something that does. The slirp plugin as a URL passed straight to connect
  gives a complete userspace gateway in one process, and the section explains
  why slirp's own DHCP server wants a link of its own.
• Filtering and priorities, including the ordering rule that catches people
  out — rules are evaluated in the reverse of the order they were added.
• When it does not work: what each management command tells you, and the
  behaviours that look like faults and are not — the first packet towards an
  unresolved next hop is dropped, ARP entries never expire, the DHCP server
  advertises a fixed resolver.

Verification

Every command in the document was run against a live router, not transcribed
from the source: the two-switch example brought up and checked, HTTP carried
between hosts on either segment through it, each ipfilter and queue line
accepted and its effect confirmed in the tables, and the DHCP server observed
handing out a lease.

Plain text, in the style of the other files in doc/, and added to
EXTRA_DIST so it installs with them.

[danielinux]

Thank you @zirize please rebase + resolve conflicts

[zirize]

Rebased onto master — the PR now carries just the HOWTO commit, and there are no conflicts left. Thanks for the review!

[danielinux]

Thank you for the contribution!