doc/main/index: Clean up menu and builtins.

There is no ubuiltins module, so we shouldn't mention it.

Also reduce size of other module titles so they fit on one line in the
menu. This makes it easier to read and get an overview.

doc/api/index.rst

@@ -74,11 +74,11 @@ findings on our `support page`_ so we can make Pybricks even better.
 
 .. toctree::
    :maxdepth: 1
-   :caption: Additional Modules
+   :caption: MicroPython Modules
    :hidden:
 
-   micropython
    ubuiltins
+   micropython
    uerrno
    uio
    umath

doc/api/ubuiltins.rst

@@ -1,4 +1,4 @@
-:mod:`ubuiltins` -- Built-in functions and exceptions
+Built-in functions and exceptions
 =====================================================
 
 .. automodule:: ubuiltins

doc/api/urandom.rst

@@ -1,4 +1,4 @@
-:mod:`urandom` -- Generate pseudo-random numbers
+:mod:`urandom` -- Pseudo-random numbers
 ================================================
 
 .. automodule:: urandom

src/ubuiltins/__init__.py

@@ -14,10 +14,7 @@
 # Copyright (c) 2001-2021 Python Software Foundation
 
 """
-All builtin functions and exceptions are described here. They are also
-available via ``ubuiltins`` module.
-
-.. tip:: Since these are built-in, you can (and should!) use them without importing.
+The following functions and exceptions can be used without importing anything.
 """
 
 from typing import (