Describe adding and configuring Flowable REST for Jmix applications with BPM-addon

[m-orlova]

Since 2.4 we will not provide the bpm-rest and bpm-rest-starter modules which attach and configure Flowable REST endpoints (see jmix/jmix-framework#2944) to Jmix application with BPM add-on.

Need to add an instruction how to add Flowable REST to the project and configures security for added endpoints. Some details are already described in the issue comment.

Also the bpm-rest module contains the only one additional endpoint to get a Jmix process form data. We can provide small instruction or notice that to implement the same endpoint we need create a custom REST endpoint and use io.jmix.bpm.processform.ProcessFormDataExtractor Spring bean to get data about Jmix process form.