Document project root behavior in git repositories

[netthier]

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Bug description

The docs for deployment.yaml path-based includes currently contain the following text:

Only directories that are part of the kluctl project are allowed.

This is also the behavior that occurs in a simple kluctl project, i.e. path-based includes referencing directories anywhere else will not work (e.g. ../).
However, as per this thread in Slack this behavior changes when the project is inside of a git repository, in which case all paths inside the git repository are allowed.
This means that in a git repository containing two projects side-by-side one of them can include the other one with a ../other-project include.
This special behavior for includes inside git repositories should be documented.

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