Update developer docs landing page

Include sections on fpm model and fpm backend

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@@ -67,6 +67,22 @@ To work with an *fpm* project as a user you can completely rely on the command l
 The command line interface is build with the [M_CLI2](https://github.com/urbanjost/M_CLI2) module and can be found in [[fpm_command_line]].
 
 
+## The package model
+
+Once front-end inputs have been received from the package manifest and command line interface, *fpm* will construct an
+internal representation of the package and its dependencies. This internal representation is known as the package *model*.
+The model and its associated data types should encapsulate all the information required to correctly build a package and
+should be independent of the intended backend build system. Information stored in the model includes: build targets and
+their inter-dependencies; compiler and compiler flags; library linking information.
+
+For more information on the contents of the package model and the process for constructing it, please see [[fpm_model]].
+
+## The build backend
+
+Once a complete package model has been constructed, it can be passed to a *backend* for either performing the compilation
+and linking of targets, or for generating configuration files for a third-party build system.
+Currently, only a native backend is implemented in *fpm*. See [[fpm_backend]] for more information.
+
 ## Generating this documentation
 
 This documentation is generated by [FORD](https://github.com/Fortran-FOSS-Programmers/FORD).