Add routing peer sizing guide (#815)

* docs: add routing peer sizing guide

Add a Sizing Routing Peers page under Networks covering the four-step
sizing method, a per-peer capacity table, the tuning levers that matter,
and how to scale out by sharding load across identical Networks.

Cross-link it from How Routing Peers Work (HA note + related tile) and
the Networks overview, and add it to the docs navigation.

* docs: refine wording in routing peer sizing guide

Generalize the Acme example to remote users, correct the encrypt/decrypt
framing and reach the local network rather than the datacenter, use
'routing peer' instead of 'gateway', and rename the recap to Summary.

* docs: tighten and correct HA behavior in routing peer sizing guide

Correct the high-availability description: a single Network does not
balance load across its peers — different metrics give failover (one
peer carries all), equal metrics give latency-based nearest-peer
selection, which splits traffic by geography but never evenly. Shard
into more Networks to split load deterministically.

Also collapse redundant restatements, drop the secondary worked
example (the capacity table covers it), and slim the commodity-hardware
guidance.

* docs: add 1- and 2-vCPU rows to the routing peer capacity table

Extend the capacity table down to 1 and 2 vCPUs, drop the 'or more' from
the interface column, and adjust the methodology note so the interface
column reads uniformly as the minimum NIC to pair with each size.

* docs: add userspace WireGuard table and when-to-use cases

Add a userspace-mode capacity table showing wireguard-go does not scale
across cores (download plateaus ~6.8 Gbps, only ~4-5 cores used), and
the cases where a routing peer runs userspace: missing/broken/conflicting
kernel module, no TUN device (netstack, incl. rootless Docker), non-Linux
peers, and forcing userspace to capture policy IDs and blocked traffic
events. Name the exact benchmark CPU (Xeon Platinum 8375C).

* docs: replace 'sharding' with plain wording in sizing guide

Rename the Scaling out heading and reword the body, description, and
recap to talk about splitting load across more Networks instead of
sharding. Update the in-page anchor link to match the new heading.

* docs: clarify download/upload direction bullets in sizing guide

Lead each direction bullet with Download:/Upload: and say the routing
peer encrypts/decrypts, tying the pulling/pushing distinction to the
capacity table's column names.

* docs: replace 'shard' with plain wording in HA note

* docs: add jumbo frames section to routing peer sizing guide

* docs: align jumbo upload figure with capacity table, mark 16-vCPU line-rate as projection

* docs: use consistent numerals for MTU byte sizes

src/components/NavigationDocs.jsx

@@ -200,6 +200,10 @@ export const docsNavigation = [
             title: 'How Routing Peers Work',
             href: '/manage/networks/how-routing-peers-work',
           },
+          {
+            title: 'Sizing Routing Peers',
+            href: '/manage/networks/sizing-routing-peers',
+          },
           {
             title: 'Masquerade',
             href: '/manage/networks/masquerade',