Document: Add meter plugin

[andig]

We need to document the new feature introduced in this PR: https://github.com/evcc-io/evcc/pull/21477

[VolkerK62]

Beispiel (Hausspeicher mit Leistungsmessung per Shelly und externem Soc):

- name: battery
  type: custom
  power:
    source: meter
    config:
      type: template
      template: shelly-1pm
      host: 192.168.178.21
      channel: 0
    method: power
    scale: -1
  energy:
    source: meter
    config:
      type: template
      template: shelly-1pm
      host: 192.168.178.21
      channel: 0
    method: energy
  soc:
    source: mqtt
    topic: Haus/Batterie
    jq: .soc
    timeout: 60s