Webdoc: describe Beam API; rm unused shortcodes

hugo/content/py/sample/multilayer.md

@@ -9,8 +9,7 @@ weight = 35
 The ***Define materials*** stance defines four materials in terms
 of their scattering length density (SLD).
 The arguments of the constructor-like global function
-[MaterialBySLD]({{% ref-api "group__materials" %}})
-are `name`, `real_sld`, `imag_sld`.
+MaterialBySLD are `name`, `real_sld`, `imag_sld`.
 Both `real_sld` and `imag_sld` are in units of inverse square angstroms.
 
 SLD values for a wide variety of materials can be found on https://sld-calculator.appspot.com