DOC: configuration: document challenge-ready and dns-delay options for ACME

DOC: configuration: document challenge-ready and dns-delay options for ACME

Add documentation for two new directives in the acme section:

- challenge-ready: configures the conditions that must be satisfied
  before notifying the ACME server that a dns-01 challenge is ready.
  Accepted values are cli, dns and none. cli waits for an operator
  to signal readiness via the "acme challenge_ready" CLI command. dns
  performs a DNS pre-check against the "default" resolvers section,
  not the authoritative name servers. When both are combined, HAProxy
  waits for the CLI confirmation before triggering the DNS check.

- dns-delay: configures the delay before the first DNS resolution
  attempt and between retries when challenge-ready includes dns.
  Default is 300 seconds.