fix(libstore/s3-binary-cache-store): include documentation from markdown file

The S3BinaryCacheStoreConfig::doc() function was returning a minimal
hardcoded 3-line string instead of including the comprehensive
documentation from s3-binary-cache-store.md.

This was introduced in PR #13752 which moved the prose documentation to
the markdown file but forgot to update the doc() function to use it.

src/libstore/s3-binary-cache-store.cc

@@ -454,11 +454,9 @@ std::string S3BinaryCacheStoreConfig::getHumanReadableURI() const
 
 std::string S3BinaryCacheStoreConfig::doc()
 {
-    return R"(
-        **Store URL format**: `s3://bucket-name`
-
-        This store allows reading and writing a binary cache stored in an AWS S3 bucket.
-    )";
+    return
+#include "s3-binary-cache-store.md"
+        ;
 }
 
 ref<Store> S3BinaryCacheStoreConfig::openStore() const