docs: Unify all the ports into one set of documentation.

With this commit there is now only one entry point into the whole
documentation, which describes the general MicroPython language, and then
from there there are links to information about specific platforms/ports.

This commit doesn't change content (almost, it does fix a few internal
links), it just reorganises things.

docs/index.rst

@@ -0,0 +1,12 @@
+MicroPython documentation and references
+========================================
+
+.. toctree::
+
+    library/index.rst
+    reference/index.rst
+    genrst/index.rst
+    license.rst
+    pyboard/quickref.rst
+    esp8266/quickref.rst
+    wipy/quickref.rst