Analog Continuous mode API (#8715)

* Adds Analog Continuous mode API

* fix when stopping/starting ADC data are not complete

* Added example

* fix size check

* update frequency in example

* set buffer to NULL if error occurs

* add docs

* set buffer to null on error

* fix example

* update docs

* fix example

* change return value to bool type

* updated adc modes description in docs

* Add empty line at the end of sketch

Author: P-R-O-C-H-Y