Add developer docs for incoming receptor mesh features (#10747)

* Add developer docs for incoming receptor mesh features

* Additional wording about the receptor mesh process

* Wrap up docs feedback changes and polishing

* Add in way more terminology introductions, delete statement about past

* Fix typo around OCP-incluster type

docs/receptor_mesh.md

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+## Receptor Mesh
+
+AWX uses a [Receptor](https://github.com/ansible/receptor) mesh to transmit "user-space" unified jobs:
+ - jobs
+ - ad hoc commands
+ - inventory updates
+
+to the node where they run.
+
+> NOTE: user-space jobs are what carry out the user's Ansible automation. These job types run inside of the designated execution environment so that the needed content is available.
+
+> NOTE: The word "node" corresponds to entries in the `Instance` database model, or the `/api/v2/instances/` endpoint, and is a machine participating in the cluster / mesh.
+
+The unified jobs API reports `controller_node` and `execution_node` fields.
+The execution node is where the job runs, and the controller node interfaces between the job and server functions.
+
+Before a job can start, the controller node prepares the `private_data_dir` needed for the job to run.
+Next, the controller node sends the data via `ansible-runner`'s `transmit`, and connects to the output stream with `process`.
+For details on these commands, see the [ansible-runner docs on remote execution](https://ansible-runner.readthedocs.io/en/latest/remote_jobs.html).
+
+On the other side, the execution node runs the job under `ansible-runner worker`.
+
+### Split of Control Plane versus Execution Plane
+
+Instances in the **control plane** run persistent AWX services (like the web server, task dispatcher, etc.), project updates, and management jobs. Instances in the **execution plane** run user-space jobs.
+
+The task manager logic will not send user-space jobs to **control-only** nodes.
+In the inventory definition, the user can set a flag to designate this node type.
+
+**Execution-only** nodes have a minimal set of software requirements needed to participate in the receptor mesh and run jobs under ansible-runner with podman isolation.
+These _only_ run user-space jobs, and may be geographically separated (with high latency) from the control plane.
+They may not even have a direct connection to the cluster, and use other receptor **hop** nodes to communicate.
+
+**Hybrid** (control & execution nodes) are instances in the control plane that are allowed to run user-space jobs.
+
+#### Receptor Configuration Work Type
+
+Execution-only nodes need to advertise the "ansible-runner" work type.
+User-space jobs are submitted as a receptor work unit with this work type.
+
+An entry like this should appear in its `receptor.conf` (receptor configuration file):
+
+```
+- work-command:
+    worktype: ansible-runner
+    command: ansible-runner
+    params: worker
+    allowruntimeparams: true
+```
+
+Control (and hybrid) nodes advertise the "local" work type instead.
+So the entry is the same as above, except that it has `worktype: local`.
+Project updates are submitted as this work type.
+If user-space jobs run on a hybrid node, they will also run as the "local" work type.
+
+Here is a listing of work types that you may encounter:
+
+ - `local` - any ansible-runner job ran in a traditional install
+ - `ansible-runner` - remote execution of user-space jobs
+ - `kubernetes-runtime-auth` - user-space jobs ran in a container group
+ - `kubernetes-incluster-auth` - project updates and management jobs on OpenShift Container Platform
+
+### Auto-discovery of execution nodes
+
+Instances in control plane must be registered by the installer via `awx-manage`
+commands like `awx-manage register_queue` or `awx-manage register_instance`.
+
+Execution-only nodes are automatically discovered after they have been configured and join the receptor mesh.
+Control nodes should see them as a "Known Node".
+
+Control nodes check the receptor network (reported via `receptorctl status`) when their heartbeat task runs.
+Nodes on the receptor network are compared against the `Instance` model in the database.
+
+If a node appears in the mesh network which is not in the database, then a "health check" is started.
+This will submit a work unit to the execution node which then outputs important node data via `ansible-runner`.
+The `capacity` field will obtain a non-zero value through this process, which is necessary to run jobs.