Apps for custom components concepts

[sfc-gh-dmatthews]

📚 Context

This PR adds app source code for embedded apps in the custom components v2 concept docs.

This also carves out an exception in the pre-commit hook so that all example code in the python example source directories use a tab of four spaces. (Code is copied between the app source and docs when writing, and linting is used to make sure it's all well-formatted. I want all the code that we show in the docs to have a consistent indent across languages--four spaces--and it's easier to avoid errors when the docs and the source are exact copies.)

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