Support per-store Markdown documentation

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
+```
+
+)"