Copy configuration documentation from pre-commit to prek

[wookie184]

Currently, it is recommended to use the pre-commit docs for details on configuration:

prek/docs/configuration.md

Lines 21 to 22 in d4adf8b

	For configuration details, refer to the official pre-commit docs:
	[pre-commit.com](https://pre-commit.com/)

This has some issues:

• It is not immediately clear what stuff is relevant to prek, or if it is 100% accurate for prek.
• If a new feature is added to pre-commit, or a change is made, it may appear there before it is implemented in prek.
• It restricts prek from making changes/additions to configuration features, or documentation improvements.

My example is that when looking for configuration for language: system and noticed that pre-commit renamed and deprecated the old naming. This made me question a) does prek support the new name, and b) is prek also deprecating the old naming (which I found out by looking at #1073).

It would be nice to copy across the relevant documentation, which would help with these issues.