Allow customising which JupyterLite apps are to be built

[agriyakhetarpal]

Problem

It is currently not possible to customise which of the apps JupyterLite has to build as a result of jupyterlite-sphinx embedding its build command (it is possible by overriding --apps via jupyterlite_build_command_config, but that's limited to advanced users). There's another option, i.e., to use jupyter_lite_config.json to do this, but this file's purpose has not been explained to the user directly.

Proposed Solution

A jupyterlite_apps configuration option hooked into Sphinx which can take a list such as ["repl", "voici", "notebook"], and so on to pass to --apps, with a UserWarning (or a RuntimeError?) emitted if a jupyter_lite_config.json file exists + already has a ["LiteBuildConfig"]["apps"] key that collides with said configuration option.

Additional context

This would be helpful for those users who willingly want to customise which apps to build for size reasons, etc. without going through the jupyter_lite_config.json route.

[jtpio]

Thanks @agriyakhetarpal.

Linking to #114 as related.