Make error modes a subsection of summary

Sections which are appending information to an existing section
looks nice in the table of contents on the sidebar.

They appear indented under the main section, in this case
summary observations.

Author: SAKavli

docs/ert/reference/configuration/observations.rst

@@ -132,7 +132,7 @@ Here are two examples:
 .. _error_modes:
 
 Error modes for summary observations
-------------------------------------
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 The item ERROR_MODE can take three different values: ABS, REL or RELMIN.
 The default error mode for the :ref:`SUMMARY_OBSERVATION <summary_observation>`