docs/develop/cmodules.rst: Document C-modules and micropython.cmake.
docs/develop/cmodules.rst: Document C-modules and micropython.cmake.

Documents the micropython.cmake file required to make user C modules
compatible with the CMake build system.

Signed-off-by: Phil Howard <e888d2bd6f13f82caa51a37c03d034c76f661ba3@pimoroni.com>

adafruit/circuitpython,bvernoux/micropython,adafruit/circuitpython,henriknelson/micropython,henriknelson/micropython,henriknelson/micropython,adafruit/circuitpython,bvernoux/micropython,henriknelson/micropython,henriknelson/micropython,bvernoux/micropython,bvernoux/micropython,adafruit/circuitpython,adafruit/circuitpython,bvernoux/micropython,adafruit/circuitpython