Using xxx_periph.h in whole IDF instead of xxx_reg.h, xxx_struct.h, xxx_channel.h ... .
Cleaned up header files from unnecessary headers (releated to soc/... headers).
1. add sigma-delta code and example.
2. add gpio output signal in io matrix.
Squashed commits:
[a25e7d4] minor changes for doc
[6a03a1e] minor changes on comments and doc.
[97dd9e4] Add doc.
update index.rst
change example index
