Documentation/platforms/avr: move documents to fix table of contents

The document context-switch-notes.rst was leaking into "Supported Platforms"
table of contents. To fix this, it was moved to separate common directory
and referenced from AVR index.rst

Similar to that, the gpio_intr_mux.rst document was leaking into list
of supported AVR families. This too was fixed by moving it to separate
docs directory.

The change was tested by building the Documentation using make html

Signed-off-by: Kerogit <kr.git@kerogit.eu>

Documentation/platforms/avr/avrdx/index.rst

@@ -110,3 +110,11 @@ Supported Boards
 
    boards/*/*
 
+Other documents
+===============
+
+.. toctree::
+   :glob:
+   :maxdepth: 1
+
+   docs/*

Documentation/platforms/avr/index.rst

@@ -8,4 +8,15 @@ The following AVR chips are supported:
    :maxdepth: 1
    :glob:
 
-   */*
+   at32uc3/*
+   at90usb/*
+   atmega/*
+   avrdx/*
+
+Other documents:
+
+.. toctree::
+   :maxdepth: 1
+   :glob:
+
+   common/*