File-Based Binary Cache Store

1st chunk of `src/libstore/local-binary-cache-store.md`

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R"(

**Store URL format**: `file://`*path*

This store allows reading and writing a binary cache stored in *path*
in the local filesystem. If *path* does not exist, it will be created.

For example, the following builds or downloads `nixpkgs#hello` into
the local store and then copies it to the binary cache in
`/tmp/binary-cache`:

```
# nix copy --to file:///tmp/binary-cache nixpkgs#hello
```

)"

Title: File-Based Binary Cache Store

Summary

Describes the `file://` store URL format for using a local filesystem directory as a binary cache. It allows Nix to read from and write to a binary cache stored at the specified path. If the path doesn't exist, it will be created. An example demonstrates copying a package to a local binary cache.