doc/main: Move signals to Pybricks TOC.

This was the only remaining section under "extras", so it is better to have it elsewhere. Since it only applies to the Pybricks package, put it there.

Also rename Pybricks Modules heading to Pybricks Package, since it behaves more like a package in contrast with the MicroPython modules that we now document as well.

doc/main/index.rst

@@ -101,7 +101,7 @@ Pybricks Documentation
 
 .. toctree::
    :maxdepth: 1
-   :caption: Pybricks Modules
+   :caption: Pybricks Package
    :hidden:
 
    hubs/index
@@ -111,6 +111,7 @@ Pybricks Documentation
    tools/index
    robotics
    geometry
+   signaltypes
 
 .. toctree::
    :maxdepth: 1
@@ -125,11 +126,3 @@ Pybricks Documentation
    micropython/urandom
    micropython/uselect
    micropython/usys
-
-.. toctree::
-   :maxdepth: 1
-   :caption: Engineering Extras
-   :hidden:
-
-   signaltypes
-..    frames