[core] Split lt_api.c into separate units

docs/contrib/lt-api.md

@@ -0,0 +1,13 @@
+# API functions guide
+
+The [LibreTiny C API](../dev/lt-api.md) functions are split between three types: common, weak and family.
+
+- Common functions are implemented in the base, common core and are the same between all families.
+- Weak functions are provided in the common core, but can (and sometimes should) be overridden by family cores. They sometimes provide usable default implementations (which *can* be overriden to provide e.g. a better way to do something), otherwise they're empty (e.g. if a family doesn't support such a feature).
+- Family functions are not provided in the common core and have to be implemented in the family core.
+
+A quick outline of all available functions and their types:
+
+{%
+	include-markdown "lt-api-functions.md"
+%}