📝 add more API documentation

doc/mkdocs/docs/api/basic_json/binary.md

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+# basic_json::binary
+
+```cpp
+// (1)
+static basic_json binary(const typename binary_t::container_type& init);
+static basic_json binary(typename binary_t::container_type&& init);
+
+// (2)
+static basic_json binary(const typename binary_t::container_type& init,
+                         std::uint8_t subtype);
+static basic_json binary(typename binary_t::container_type&& init,
+                         std::uint8_t subtype);
+```
+
+1. Creates a JSON binary array value from a given binary container.
+2. Creates a JSON binary array value from a given binary container with subtype.
+ 
+Binary values are part of various binary formats, such as CBOR, MessagePack, and BSON. This constructor is used to
+create a value for serialization to those formats.
+
+## Parameters
+
+`init` (in)
+:   container containing bytes to use as binary type
+
+`subtype` (in)
+:   subtype to use in CBOR, MessagePack, and BSON
+
+## Return value
+
+JSON binary array value
+
+## Exception safety
+
+Strong guarantee: if an exception is thrown, there are no changes in the JSON value.
+
+## Complexity
+
+Linear in the size of `init`; constant for `typename binary_t::container_type&& init` versions.
+
+## Notes
+
+Note, this function exists because of the difficulty in correctly specifying the correct template overload in the
+standard value ctor, as both JSON arrays and JSON binary arrays are backed with some form of a `std::vector`. Because
+JSON binary arrays are a non-standard extension it was decided that it would be best to prevent automatic initialization
+of a binary array type, for backwards compatibility and so it does not happen on accident.
+
+## Version history
+
+- Added in version 3.8.0.