bootstrapping overview: add an image (#1902)

Summary:
Pull Request resolved: #1902

feedback from yesterday's presentation says we need more of this sort of thing so making a start here: add a figure to the bootstrapping-overview page.

Reviewed By: johnwhumphreys

Differential Revision: D87146992

fbshipit-source-id: 1bdd68a1c525b5cd7f63eea1709135f1c5b5443c

Author: shayne-fletcher

docs/source/books/hyperactor-mesh-book/src/meshes/bootstrapping-overview.md

@@ -1,10 +1,29 @@
 # Bootstrapping Overview
 
-This section explains one concrete, working way to bring up a mesh — the shape shown in the unit test `bootstrap_cannonical_simple`. The later pages break it into layers:
+*One sentence:* the controller boots remote hosts, then spawns procs (**1 proc = 1 OS process**). Each host runs one **HostMeshAgent**; **every proc runs its own ProcMeshAgent** supervising that proc’s user/service actors.
 
-1. get a proc + instance (control endpoint)
-2. use the v0 process allocator and bootstrap handshake to get remote runtimes
-3. turn those runtimes into real hosts
-4. then spawn procs and actors on those hosts
+<figure id="fig-arch" style="text-align:center;">
+  <img src="image/mesh-elements.png"
+       alt="High-level architecture"
+       style="width:60%; display:block; margin:0 auto;">
+  <figcaption><strong>Figure 1.</strong> High-level architecture of the host, procs, and agents.</figcaption>
+  <p style="text-align:center; margin-top:0.25rem;">
+    Legend: ◻︎ = proc · ○ = actor (service or user).
+  </p>
+</figure>
 
-For the full, runnable test, see the appendix.
+## What you’re looking at
+
+- **Controller process** (left): owns orchestration APIs and kicks off bootstrapping.
+- **HostMeshAgent** (one per host): manages host-local resources and the service proc.
+- **ProcMeshAgent** (one per proc): supervises that proc’s actors (service + user).
+- **Procs** (squares): runtime containers; *1 proc = 1 OS process*.
+
+## Boot sequence (10k-ft)
+
+1. Obtain a **proc + instance** (control endpoint).
+2. Use the **v0 process allocator** and **bootstrap handshake** to get remote runtimes.
+3. Promote those runtimes into real **hosts** (start HostMeshAgent).
+4. **Spawn procs and actors** on those hosts (start proc/ProcMeshAgent, spawn  actors).
+
+_For the full, runnable test (see `bootstrap_cannonical_simple`), see the appendix._