EUCLIDPCR-1863 Include documentation in the toctree

Author: Jorge Fernandez Hernandez

docs/index.rst

@@ -256,6 +256,7 @@ The following modules have been completed using a common API:
   cadc/cadc.rst
   casda/casda.rst
   linelists/cdms/cdms.rst
+  esa/euclid/euclid.rst
   esa/hsa/hsa.rst
   esa/hubble/hubble.rst
   esa/integral/integral.rst