Document the control plane bootstrap process

[johananl]

Lokomotive uses a self-hosted control plane based on Bootkube + Helm. The sequence of events which eventually results in an operational self-hosted k8s control plane is rather complex and contains multiple potential failure points. Therefore, it would be very useful to have document (possibly under the "concepts" category) which describes the various "layers" which comprise the control plane as well as the sequence of steps which take place from the moment a controller machine is created until the self-hosted control plane is converged.

I suggest we include the following in the document:

• A general explanation about what a self-hosted control plane is.
• A diagram showing the Lokomotive control plane architecture.
• A step-by-step explanation of a successful control plane bootstrap process.
• A troubleshooting section which explains what to look at in case the control plane on a cluster can't converge.
• Any "gotchas", e.g. special things to note during cluster failures, cluster upgrades etc.

[invidian]

Note, if #297 gets merged, we should document it too.