Docs: cover how to deploy and run the built environments

[ncoghlan]

Currently the only docs on actually deploying and using the built environments are the list of layer config fields in https://venvstacks.lmstudio.ai/file-formats/#deployed-layer-configuration

Summarising my notes on this from the Discord channel:

1. Unpack all 3 layers in the same location on the target system (so the directories end up next to each other). For each one, run the post-installation script after unpacking.
2. Once everything is unpacked, run the application layer's launch module using the -m switch for the Python executable in that environment.

Running the post-installation script when unpacking each layer:

1. Read the layer config from {env_path}share/venv/metadata/venvstacks_layer.json
2. Resolve base_python from the layer config relative to the environment folder
3. Use that resolved path to run {base_python_path} {env_path}/postinstall.py

Running the launch module for application layers:

1. Read the layer config from {env_path}share/venv/metadata/venvstacks_layer.json
2. Read launch_module from the layer config
3. Resolve python from the layer config relative to the environment folder
4. Use that resolved path to run {python_path} -m {launch_module} (potentially with CLI arguments, depending on the use case)

[ncoghlan]

There's a lot more to potentially cover, like using layer versioning and the published metadata files to allow the embedding app to make informed decisions about whether it already has a given layer installed, and doesn't need to download it again.

That will be tricky to write before #89 is resolved, though, and is out of scope for this particular issue report. This issue just covers the basic case of "I have built a set of layers and want to deploy them to a target machine"

[ncoghlan]

In the absence of #35 (which suggests turning the docs example into a fully implemented demo), it would also be good to have a worked example of building and deploying the sample project from the test suite.

Both the "assumed knowledge" version (with exact layer version numbers updated as needed):

$ cpython-3.11/bin/python cpython-3.11/postinstall.py
$ cpython-3.11/bin/python framework-scipy@1/postinstall.py
$ cpython-3.11/bin/python app-scipy@1/postinstall.py
$ app-scipy@1/bin/python -m scipy_import

And the platform-and-application independent version that reads the metadata config file.

[ncoghlan]

I've put "good first issue" on this, mainly because "What do you wish the docs had told you before you started?" is a genuinely useful viewpoint here (but also because this doesn't require any deep knowledge of venvstacks internals, just a basic grasp on how to use it).