[Doc] introduction to prj file docu

Documentation/ProjectFile/i_ProjectFile.md

-Entry point for project file parameter documentation
+The OGS6 input file parameters are documented in the page hierarchy rooted here.
+
+Depending on the type of the parameters the corresponding page titles have
+different prefixes, namely:
+
+ - <b>[tag] </b> if the parameter is an XML tag in the input file<br>
+   Example: \ref ogs_file_param__prj__linear_solvers
+ - <b>[attr]</b> if the parameter is an attribute of an XML tag in the input
+   file<br>
+   Example: \ref ogs_file_attr__gml__points__point__x
+ - <b>[case]</b> either on the top level of the documentation tree, or to
+   distinguish between different cases (i.e., sets of configuration options,
+   which usually also means different underlying C++ types) in the input
+   file.<br>
+   Usually one can choose one of several cases by specifying the associated
+   <tt>&lt;type&gt;</tt> tag in the input file.<br>
+   Example: \ref ogs_file_param__linear_solver
+   and \ref ogs_file_param__parameter__Constant (<tt>&lt;type&gt;Constant&lt;/type&gt;</tt>)
+   vs. \ref ogs_file_param__parameter__MeshProperty (<tt>&lt;type&gt;MeshProperty&lt;/type&gt;</tt>)
+
+The input file parameters are documented within a tree structure (cf. the
+navigation tree on the left). This structure resembles the XML document tree of
+the input files, however, there are some small differences (see below).
+
+Currently two different input files are documented:
+The project file (\ref ogs_file_param__prj "prj")
+and the geometry file (\ref ogs_file_param__gml "gml").
+All other cases on the top level do not represent separate input files but
+rather are shortcuts to certain project file sections, which are provided only in
+order to keep the documentation tree flat, they have no other special meaning.
+
+A path in the documentation tree corresponds to a path in the prj or gml input
+file, e.g., \ref ogs_file_param__prj__process_variables__process_variable__boundary_conditions__boundary_condition
+corresponds to the path <tt>process_variables.process_variable.boundary_conditions.boundary_condition</tt>
+in the project file (here parent and child tags are separated by a dot).<br>
+Note: The top level XML tags (i.e., <tt>&lt;OpenGeoSysProject&gt;</tt> and <tt>&lt;OpenGeoSysGLI&gt;</tt>)
+have been omitted in the entire documentation for the sake of brevity.
+
+There are two exceptions to this rule, both related to the <em>[case]</em>
+prefix:
+ 1. Cases at the top level do not translate to XML tags. The cases
+    \ref ogs_file_param__prj and \ref ogs_file_param__gml mark the root of the
+    prj and gml input file, respectively.
+    All other top level cases, however, belong somewhere in the project file XML tree.
+    You can see where they belong in the <em>Additional Info</em> section of the [tag] and
+    [attr] pages, e.g., \ref ogs_file_param__boundary_condition__geometry of the
+    boundary condition has the full XML tag path
+    <tt>process_variables.process_variable.boundary_conditions.boundary_condition.geometry</tt>.
+ 2. Cases that distinguish types do not translate to XML tags either.
+    They enclose a set of configuration settings that can be used at that specific point.<br>
+    Example: There is a \ref ogs_file_param__initial_condition__MeshProperty "MeshProperty"
+    initial condition, which has a
+    \ref ogs_file_param__initial_condition__MeshProperty__field_name "field_name" tag.
+    I.e., if you select the MeshProperty IC, you can (or must) specify the XML tag
+    with the path <tt>process_variables.process_variable.initial_condition.field_name</tt>
+    in the project file.<br>
+    The MeshProperty IC can be selected by setting the
+    \ref ogs_file_param__initial_condition__type "process_variables.process_variable.initial_condition.type" tag to
+    <tt>MeshProperty</tt>.
 
 # Further Information