docs: describe sidecar channels
With this commit we add a first description and HOWTO about what sidecar
channels are and how they can be created.
We add this commit first to give any reviewer the full context about
what we're trying to achieve first.
* [Orders](orders.md)
* [Channel Leases](channel_leases.md)
* [Batch Execution](batch_execution.md)
+* [Sidecar Channels](sidecar_channels.md)
* [FAQs](faq.md)
