docs: Fix Sphinx 3.x warnings, and enable warnings-as-errors on build.

This enables warnings as errors and fixes all current errors, namely:

- reference to terms in the glossary must now be explicit (:term:)
- method overloads must not be declared as a separate method or must
  use :noindex:
- 2 cases where `` should have been used instead of `

docs/library/micropython.rst

@@ -100,7 +100,7 @@ Functions
    unlocked.
 
    Note: `heap_locked()` is not enabled on most ports by default,
-   requires `MICROPY_PY_MICROPYTHON_HEAP_LOCKED`.
+   requires ``MICROPY_PY_MICROPYTHON_HEAP_LOCKED``.
 
 .. function:: kbd_intr(chr)