Update MPS doc to correct file names, commands (#12772)

### Summary
There are a few updates needed to the MPS docs. I made the following
changes:
* Link to the build from source docs, as the instructions on the MPS
docs have to be run from the source directory.
* Update the MV3 pte file name is some of the example commands. It
previously did not match the file generated from the example export.
* Update the devtools inspector command to use the devtools namespace
instead of sdk, which no longer works.

### Test plan
Ran through the MPS documentation page locally with the updated
commands.

Author: GregoryComer

docs/source/backends-mps.md

@@ -40,7 +40,9 @@ In order to be able to successfully build and run a model using the MPS backend
 
 ## Setting up Developer Environment
 
-***Step 1.*** Please finish tutorial [Getting Started](getting-started.md).
+***Step 1.*** Complete the steps in [Getting Started](getting-started.md) to set up the ExecuTorch development environment.
+
+You will also need a local clone of the ExecuTorch repository. See [Building ExecuTorch from Source](using-executorch-building-from-source.html) for instructions. All commands in this document should be run from the executorch repository.
 
 ## Build
 
@@ -70,12 +72,12 @@ cd executorch
 ## Run the mv3 generated model using the mps_executor_runner
 
 ```bash
-./cmake-out/examples/apple/mps/mps_executor_runner --model_path mv3_mps_bundled_fp16.pte --bundled_program
+./cmake-out/examples/apple/mps/mps_executor_runner --model_path mv3_mps_float16_bundled.pte --bundled_program
 ```
 
 - You should see the following results. Note that no output file will be generated in this example:
 ```
-I 00:00:00.003290 executorch:mps_executor_runner.mm:286] Model file mv3_mps_bundled_fp16.pte is loaded.
+I 00:00:00.003290 executorch:mps_executor_runner.mm:286] Model file mv3_mps_float16_bundled.pte is loaded.
 I 00:00:00.003306 executorch:mps_executor_runner.mm:292] Program methods: 1
 I 00:00:00.003308 executorch:mps_executor_runner.mm:294] Running method forward
 I 00:00:00.003311 executorch:mps_executor_runner.mm:349] Setting up non-const buffer 1, size 606112.
@@ -112,11 +114,11 @@ python3 -m examples.apple.mps.scripts.mps_example --model_name="mv3" --generate_
 ```
 2. Run your Program on the ExecuTorch runtime and generate an [ETDump](etdump.md).
 ```
-./cmake-out/examples/apple/mps/mps_executor_runner --model_path mv3_mps_bundled_fp16.pte --bundled_program --dump-outputs
+./cmake-out/examples/apple/mps/mps_executor_runner --model_path mv3_mps_float16_bundled.pte --bundled_program --dump-outputs
 ```
 3. Create an instance of the Inspector API by passing in the ETDump you have sourced from the runtime along with the optionally generated ETRecord from step 1.
 ```bash
-python3 -m sdk.inspector.inspector_cli --etdump_path etdump.etdp --etrecord_path etrecord.bin
+python3 -m devtools.inspector.inspector_cli --etdump_path etdump.etdp --etrecord_path etrecord.bin
 ```
 
 ## Deploying and Running on Device