added read observer and encoder angle functions

[hackeybabel]

• Read observer angle
• Read encoder angle
• Read observer error

[vedderb]

One of the lines has a syntax error and the extensions also need to be registered. It is also good to update the documentation when adding new extensions.

I made an attempt here to add all position sources and documenting them: https://github.com/vedderb/bldc/commit/523e42b829b435549f19025ced9f6e9c84f37554

Can you try if that works?