add API in docs

Author: TorkelE

docs/src/api/catalyst_api.md

@@ -280,6 +280,17 @@ Base.convert
 ModelingToolkit.structural_simplify
 ```
 
+## Chemistry-related functionalities
+Various functionalities primarily relevant to modelling of chemical systems (but potentially also in biology). 
+```@docs
+@compound
+@compounds
+iscompound
+components
+coefficients
+component_coefficients
+```
+
 ## Unit validation
 ```@docs
 validate(rx::Reaction; info::String = "")