doc: add reference manual

docs/content.en/docs/reference/toml_fwd.md

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+title = "toml_fwd.hpp"
+type  = "docs"
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+
+# toml_fwd.hpp
+
+`toml_fwd.hpp` contains forward declarations of structures defined in toml11 and macro definitions.
+
+When only forward declarations of toml11 structures are needed and implementation is not required, including `toml_fwd.hpp` instead of `toml.hpp` can reduce compilation time.
+
+{{<hint warning>}}
+
+Since this file only contains forward declarations, you cannot use `toml::table`, defined as `toml::basic_value<toml::type_config>::table_type`, and similarly defined `toml::array`. This is because they require the implementation of `basic_value`.
+
+{{</hint>}}
+
+This header file and `toml.hpp` are located under `${TOML11_INCLUDE_DIR}/`, while other header files are located under `${TOML11_INCLUDE_DIR}/toml11/`.