docs: add Enterprise Commercial License overview page (#869)

* docs: add Enterprise Commercial License overview page

Add a public, shareable overview of the NetBird Enterprise Commercial
License for teams evaluating self-hosted NetBird. Answers the questions
prospects ask most: in-place migration from the open source Community
Edition, zero-downtime control-plane upgrades via active-active HA,
control-plane behavior at scale, single-tenant boundaries and the
options for serving multiple customers, and how evaluation works.

Clarifies that the Cloud "Business plan" and the self-hosted commercial
license are different products, and lists what the license unlocks
(HA, SCIM, EDR/MDM integrations, traffic-flow logging, standard support).

Served at /selfhosted/enterprise and linked from the Self-Host sidebar.

* docs: qualify connection continuity by deployment topology

The single-server upgrade answer claimed all established connections
survive a restart. That holds only when Relay runs externally. In the
default combined deployment, netbird-server bundles Management, Signal,
and Relay, so recreating it restarts Relay and active relayed sessions
reconnect. Clarify that direct peer-to-peer connections continue either
way, while relayed-session continuity depends on whether Relay is
external or restarted with the combined server.

* docs: describe the commercial PoC as assisted, with 30-day default

"Managed proof of concept" overstated the offer. Per the EULA the
customer installs and runs the stack, with NetBird providing the license
and guidance, and a commercial PoC runs 30 days by default. Reword to
"assisted proof of concept" and state the default duration.

src/components/NavigationDocs.jsx

@@ -540,6 +540,7 @@ export const docsNavigation = [
         links: [
             { title: 'Quickstart Guide', href: '/selfhosted/selfhosted-quickstart' },
             { title: 'Automated Setup', href: '/selfhosted/automated-setup' },
+            { title: 'Commercial License', href: '/selfhosted/enterprise' },
             {
                 title: 'Infrastructure as Code',
                 isOpen: false,