Document `FOUNDRY_DIR` and `XDG_` features

PR https://github.com/foundry-rs/foundry/pull/4738 added support for `FOUNDRY_DIR` and `XDG_DATA_HOME`. However, this feature is not documented in the Foundry Book. Searching for `FOUNDRY_DIR` doesn't yield any result, and `XDG_DATA_HOME` is only mentioned in the context of the [`svm`](https://getfoundry.sh/misc/faq#im-getting-solc-errors) (Solidity Version Manager).