feat(docs): add Sandbox PXE proving doc (#9815)

Adds a doc with info about how to enable client side proving in the
sandbox.

closes AztecProtocol/dev-rel#442

---------

Co-authored-by: saleel <saleel@aztecprotocol.com>

docs/docs/guides/developer_guides/local_env/sandbox_proving.md

@@ -0,0 +1,38 @@
+---
+title: Sandbox PXE Proving
+tags: [sandbox, PXE]
+---
+
+The Sandbox does not have client-side proving in the PXE enabled by default. This reduces testing times and increases development speed by allowing for rapid iteration.
+
+You may want to enable client-side proving in the Sandbox to better understand how long it takes to execute Aztec transactions.
+
+:::note
+
+Running the sandbox with client-side proving is much slower and should only be used sparingly to analyze real proving times of executing private functions of a contract.
+
+:::
+
+## Config
+
+To enable client-side proving, update the docker compose file found at `~/.aztec/docker-compose.sandbox.yml` and add the following configuration settings under the `aztec:environment` scope.
+
+```yml
+BB_BINARY_PATH: /usr/src/barretenberg/cpp/build/bin/bb
+PXE_PROVER_ENABLED: 1
+BB_WORKING_DIRECTORY: ~/bb-temp    #any temp directory for writing circuit artifacts
+```
+
+## Usage
+
+Once the `docker-compose.sandbox.yml` has been updated, restart the sandbox with:
+
+```bash
+aztec start --sandbox
+```
+
+The sandbox will take much longer to start. The first time it starts, it will need to download a large crs file, which can take several minutes even on a fast internet connection. This is a one-time operation, you will not need to download it again until you update to a new Aztec version.
+
+The sandbox will also deploy 3 Schnorr account contracts on startup. The sandbox will need to generate transaction proofs for deployment, which will take additional time.
+
+Once everything has been set up, you will see that the PXE is listening on `localhost:8080` as you would see with the sandbox running in the default mode. At this point you can use the sandbox as you would without client-side proving enabled.