Documentation for `victron` component

[nagyrobi]

Description:

Work in progress for https://github.com/ssieb/custom_components/tree/master/components/victron

Related issue (if applicable): fixes

Pull request in esphome with YAML changes (if applicable): esphome/esphome#<esphome PR number goes here>

Checklist:

• [x] Branch: next is for changes and new documentation that will go public with the next ESPHome release. Fixes, changes and adjustments for the current release should be created against current.
• [ ] Link added in /index.rst when creating new documents for new components or cookbook.