Add SD logging and update docs

- Add optional microSD CSV logging per sender/day on receiver
- Wire logger into receiver packet handling
- Document new batch header fields, build envs, and SD logging
- Make sender links open in a new tab

src/sd_logger.h

@@ -0,0 +1,7 @@
+#pragma once
+
+#include "data_model.h"
+
+void sd_logger_init();
+bool sd_logger_is_ready();
+void sd_logger_log_sample(const MeterData &data, bool include_error_text);