docs(hydro_lang): add docs for processes (#1671)

docs/docs/hydro/locations/processes.md

@@ -3,8 +3,41 @@ sidebar_position: 0
 ---
 
 # Processes
-:::caution
+The simplest type of location in Hydro is a **process**. A process represents a **single thread** running a piece of a Hydro program (with no internal concurrency). When creating a process, you can pass in a type parameter that will be used as a marker to distinguish that process from others (and will also be used to mark logs originating at that process). For example, you can create a process with a marker of `Leader` to represent a leader in a distributed system:
 
-The Hydro documentation is currently under active development! This page is a placeholder for future content.
+```rust,no_run
+# use hydro_lang::*;
+struct Leader {}
+
+let flow = FlowBuilder::new();
+let leader: Process<Leader> = flow.process::<Leader>();
+```
+
+:::note
+
+Currently, each Hydro process is deployed as a **separate** operating system process. In the future, we plan to support running multiple Hydro processes in a single operating system process for more efficient resource sharing.
 
 :::
+
+Once we have a process, we can create live collections on that process (see [Live Collections](../live-collections/index.md) for more details). For example, we can create a stream of integers on the leader process:
+
+```rust,no_run
+# use hydro_lang::*;
+# struct Leader {}
+# let flow = FlowBuilder::new();
+# let leader: Process<Leader> = flow.process::<Leader>();
+let numbers = leader.source_iter(q!(vec![1, 2, 3, 4]));
+```
+
+## Networking
+Because a process represents a single machine, it is straightforward to send data to and from a process. For example, we can send a stream of integers from the leader process to another process using the `send_bincode` method (which uses [bincode](https://docs.rs/bincode/latest/bincode/) as a serialization format). This automatically sets up network senders and receivers on the two processes.
+
+```rust,no_run
+# use hydro_lang::*;
+# struct Leader {}
+# let flow = FlowBuilder::new();
+# let leader: Process<Leader> = flow.process::<Leader>();
+let numbers = leader.source_iter(q!(vec![1, 2, 3, 4]));
+let process2: Process<()> = flow.process::<()>();
+let on_p2: Stream<_, Process<()>, _> = numbers.send_bincode(&process2);
+```