Fix of Supported platforms Table of Contents in Documentation

[kr...@kerogit.eu]

Hello,

while working on documentation for AVR DA/DB chips, I noticed that two 
documents related to networking in the simulator architecture are 
leaking to the Table of Contents for supported platforms. I prepared a 
patch to fix this - it is attached to this message as 
0001a-Documentation-platforms-fix-sim-network-support-docu.patch

The patch simply changes the glob in platforms/index.rst to only include 
index.rst from subdirectories. The documents related to networking are 
no longer included in the Table of Contents this way. I tested this 
change using make html and it seemed that nothing else was affected.

Since I don't have enough knowledge to properly determine if there are 
indeed no unwanted side effects of this change, I also created and 
alternative patch attached as 
0001b-Documentation-platforms-fix-sim-network-support-docu.patch . This 
variant fixes the TOC the same way as was done for documents related to 
support of AVR chips, where I made the same mistake and they also leaked 
into the TOC. Instead of changing platforms/index.rst, this patch moves 
the networking documents to subdirectory and changes 
platforms/sim/index.rst to load them from there.

Again, tested it using make html and it also seemed that nothing else 
was affected.

Strictly speaking, both patches can be applied but they are meant to be 
either-or. In my opinion patch variant A is better since it prevents the 
problem from happening again when someone else adds more documents. 
However, since I am not knowledgeable enough to make a judgement on 
this, I am posting both variants.

The patches are also available in a git repository nuttx.git at 
git.kerogit.eu accessible through HTTP/S. (Trying to prevent bot traffic 
by not posting the URL in machine-readable form.) The relevant branches 
are called doc_fix1 (variant A) and doc_fix1b (variant B.) I do not have 
a GitHub account so this relies on someone else to open a PR, if 
interested.