docs(automation): document driving the web build

Describe the two JavaScript functions the web build exposes
(ocs_control_submit / ocs_control_take), how to poll them, the queue and reply
limits, opening a drawing from bytes with data_base64, and closing the startup
dialog before new. Everything is taken from src/app/control/mod.rs,
src/app/startup.rs and docs/automation/web_save_smoke.cjs.

docs/automation/README.md

@@ -156,3 +156,5 @@ The repeatable live-editor evaluation draws three isolated entities in one batch
 ```sh
 python3 docs/automation/mcp_eval.py target/debug/OpenCADStudio
 ```
+
+The web build is driven through two JavaScript functions instead of a process; see [web.md](web.md).