[doc] Added documentation for new prj file setting

Documentation/ProjectFile/material/solid/constitutive_relation/MFront/library/a_path_is_relative_to_prj_file.md

+This setting modifies where the shared library containing the MFront behaviour will be looked for.
+
+The default setting of `true` should work for almost all use cases. `false` is used for some special tests in OGS's test suite.
+
+See also the documentation of the \ref ogs_file_param__material__solid__constitutive_relation__MFront__library "<code>&lt;library&gt;</code>" tag.

Documentation/ProjectFile/material/solid/constitutive_relation/MFront/library/i_library.md

+Path of the shared library containing the material model.
+
+Relative paths are interpreted relative to the project directory unless \ref ogs_file_attr__material__solid__constitutive_relation__MFront__library__path_is_relative_to_prj_file "<code>path_is_relative_to_prj_file</code>" is set to `false`, in which case the library path/name is used verbatim and will be searched based on the operating system's library lookup rules.

Documentation/ProjectFile/material/solid/constitutive_relation/MFront/t_library.md

-Path of the shared library containing the material model.
-
-Relative paths are interpreted relative to the project directory.