reference/index: Rewrite introduction paragraph to avoid confusion.

The old intro talked about "differences", but there were hardly any
sections describing differences, mostly MicroPython specific features.
On the other hand, we now have real "differences" chapter, though it's
mostly concerned with stdlib differences.

So, try to avoid confusion by changing wording and linking to the other
chapters and contrasting them with what is described in "MicroPython
language".

docs/differences/index_template.txt

@@ -1,3 +1,5 @@
+.. _cpython_diffs:
+
 MicroPython differences from CPython
 ====================================