Add renewal documentation for Traefik

[alanchrt]

We currently don't show content on renewals, but when we do (a large project), we should add an ACME renewal flow for Traefik:

---

Traefik is a modern reverse-proxy with integrated support for ACME. It's easy to get a certificate from Let's Encrypt andy other ACME compatible CAs like step-ca in Traefik, using the tls-alpn-01 ACME challenge type.

Most importantly, Traefik will need to trust your root CA certificate. Either use the LEGO_CA_CERTIFICATES environment variable to provide the full path to your {{ ca_cert }} when running Traefik, or install your root certificate in your system's trust store.

In your Traefik static configuration, you'll need to add a certificatesResolvers block:

[certificatesResolvers]
  [certificatesResolvers.myresolver]
    [certificatesResolvers.myresolver.acme]
      caServer = "https://step-ca.internal/acme/acme/directory"
      email = "anna@example.com"
      storage = "acme.json"
      tlsChallenge = true

Then, when you add routers to your dynamic configuration for HTTPS traffic, you need to set tls and tls.certResolver:

## Dynamic configuration
[http]
  [http.routers]
    [http.routers.router1]
      ...
      [http.routers.router1.tls]
        certResolver = "myresolver"

cc @tashian

[alanchrt]

Nvm, going to wire up a simple version for now.