docs/develop: Add MicroPython Internals chapter.

This commit adds many new sections to the existing "Developing and building
MicroPython" chapter to make it all about the internals of MicroPython.

This work was done as part of Google's Season of Docs 2020.

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+.. _internals_library:
+
+Implementing a Module
+=====================
+
+This chapter details how to implement a core module in MicroPython.
+MicroPython modules can be one of the following:
+
+- Built-in module: A general module that is be part of the MicroPython repository.
+- User module: A module that is useful for your specific project that you maintain
+  in your own repository or private codebase.
+- Dynamic module: A module that can be deployed and imported at runtime to your device.
+
+A module in MicroPython can be implemented in one of the following locations:
+
+- py/: A core library that mirrors core CPython functionality.
+- extmod/: A CPython or MicroPython-specific module that is shared across multiple ports.
+- ports/<port>/: A port-specific module.
+
+.. note::
+   This chapter describes modules implemented in ``py/`` or core modules.
+   See :ref:`extendingmicropython` for details on implementing an external module.
+   For details on port-specific modules, see :ref:`porting_to_a_board`.
+
+Implementing a core module
+--------------------------
+
+Like CPython, MicroPython has core builtin modules that can be accessed through import statements.
+An example is the ``gc`` module discussed in :ref:`memorymanagement`.
+
+.. code-block:: bash
+
+   >>> import gc
+   >>> gc.enable()
+   >>>
+
+MicroPython has several other builtin standard/core modules like ``io``, ``uarray`` etc.
+Adding a new core module involves several modifications.
+
+First, create the ``C`` file in the ``py/`` directory. In this example we are adding a
+hypothetical new module ``subsystem`` in the file ``modsubsystem.c``:
+
+.. code-block:: c
+
+   #include "py/builtin.h"
+   #include "py/runtime.h"
+
+   #if MICROPY_PY_SUBSYSTEM
+
+   // info()
+   STATIC mp_obj_t py_subsystem_info(void) {
+       return MP_OBJ_NEW_SMALL_INT(42);
+   }
+   MP_DEFINE_CONST_FUN_OBJ_0(subsystem_info_obj, py_subsystem_info);
+
+   STATIC const mp_rom_map_elem_t mp_module_subsystem_globals_table[] = {
+       { MP_ROM_QSTR(MP_QSTR___name__), MP_ROM_QSTR(MP_QSTR_subsystem) },
+       { MP_ROM_QSTR(MP_QSTR_info), MP_ROM_PTR(&subsystem_info_obj) },
+   };
+   STATIC MP_DEFINE_CONST_DICT(mp_module_subsystem_globals, mp_module_subsystem_globals_table);
+
+   const mp_obj_module_t mp_module_subsystem = {
+       .base = { &mp_type_module },
+       .globals = (mp_obj_dict_t *)&mp_module_subsystem_globals,
+   };
+
+   MP_REGISTER_MODULE(MP_QSTR_subsystem, mp_module_subsystem, MICROPY_PY_SUBSYSTEM);
+
+   #endif
+
+The implementation includes a definition of all functions related to the module and adds the
+functions to the module's global table in ``mp_module_subsystem_globals_table``. It also
+creates the module object with ``mp_module_subsystem``.  The module is then registered with
+the wider system via the ``MP_REGISTER_MODULE`` macro.
+
+After building and running the modified MicroPython, the module should now be importable:
+
+.. code-block:: bash
+
+   >>> import subsystem
+   >>> subsystem.info()
+   42
+   >>>
+
+Our ``info()`` function currently returns just a single number but can be extended
+to do anything.  Similarly, more functions can be added to this new module.