Add documentation about the sampling rate, serial and gain

[sebromero]

This PR adds some code documentation on how to use the PDM with the BLE Sense and the Portenta Vision Shield.

[github-actions]

Memory usage change @ e21fc6f

Board	flash	%	RAM for global variables	%
arduino:mbed:envie_m4	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:mbed:envie_m7	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:mbed:nano33ble	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0

Click for full report table

Board	libraries/Scheduler/examples/MultipleBlinks flash	%	libraries/Scheduler/examples/MultipleBlinks RAM for global variables	%	libraries/doom/examples/Doom flash	%	libraries/doom/examples/Doom RAM for global variables	%	libraries/KernelDebug/examples/KernelDebug flash	%	libraries/KernelDebug/examples/KernelDebug RAM for global variables	%	libraries/Portenta_SDCARD/examples/TestSDCARD flash	%	libraries/Portenta_SDCARD/examples/TestSDCARD RAM for global variables	%	libraries/Portenta_Video/examples/Envie_video_coreboot flash	%	libraries/Portenta_Video/examples/Envie_video_coreboot RAM for global variables	%	libraries/Portenta_System/examples/PortentaH7_getBootloaderInfo flash	%	libraries/Portenta_System/examples/PortentaH7_getBootloaderInfo RAM for global variables	%	libraries/Portenta_System/examples/PortentaH7_updateBootloader flash	%	libraries/Portenta_System/examples/PortentaH7_updateBootloader RAM for global variables	%	libraries/ThreadDebug/examples/ThreadDebug flash	%	libraries/ThreadDebug/examples/ThreadDebug RAM for global variables	%	libraries/USBHOST/examples/KeyboardController flash	%	libraries/USBHOST/examples/KeyboardController RAM for global variables	%	libraries/USBHOST/examples/Shell flash	%	libraries/USBHOST/examples/Shell RAM for global variables	%	libraries/WiFi/examples/PortentaWiFiFirmwareUpdater flash	%	libraries/WiFi/examples/PortentaWiFiFirmwareUpdater RAM for global variables	%	libraries/PDM/examples/PDMSerialPlotter flash	%	libraries/PDM/examples/PDMSerialPlotter RAM for global variables	%
arduino:mbed:envie_m4	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0
arduino:mbed:envie_m7	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0
arduino:mbed:nano33ble	0	0.0	0	0.0																									0	0.0	0	0.0													0	0.0	0	0.0

Click for full report CSV

Board,libraries/Scheduler/examples/MultipleBlinks<br>flash,%,libraries/Scheduler/examples/MultipleBlinks<br>RAM for global variables,%,libraries/doom/examples/Doom<br>flash,%,libraries/doom/examples/Doom<br>RAM for global variables,%,libraries/KernelDebug/examples/KernelDebug<br>flash,%,libraries/KernelDebug/examples/KernelDebug<br>RAM for global variables,%,libraries/Portenta_SDCARD/examples/TestSDCARD<br>flash,%,libraries/Portenta_SDCARD/examples/TestSDCARD<br>RAM for global variables,%,libraries/Portenta_Video/examples/Envie_video_coreboot<br>flash,%,libraries/Portenta_Video/examples/Envie_video_coreboot<br>RAM for global variables,%,libraries/Portenta_System/examples/PortentaH7_getBootloaderInfo<br>flash,%,libraries/Portenta_System/examples/PortentaH7_getBootloaderInfo<br>RAM for global variables,%,libraries/Portenta_System/examples/PortentaH7_updateBootloader<br>flash,%,libraries/Portenta_System/examples/PortentaH7_updateBootloader<br>RAM for global variables,%,libraries/ThreadDebug/examples/ThreadDebug<br>flash,%,libraries/ThreadDebug/examples/ThreadDebug<br>RAM for global variables,%,libraries/USBHOST/examples/KeyboardController<br>flash,%,libraries/USBHOST/examples/KeyboardController<br>RAM for global variables,%,libraries/USBHOST/examples/Shell<br>flash,%,libraries/USBHOST/examples/Shell<br>RAM for global variables,%,libraries/WiFi/examples/PortentaWiFiFirmwareUpdater<br>flash,%,libraries/WiFi/examples/PortentaWiFiFirmwareUpdater<br>RAM for global variables,%,libraries/PDM/examples/PDMSerialPlotter<br>flash,%,libraries/PDM/examples/PDMSerialPlotter<br>RAM for global variables,%
arduino:mbed:envie_m4,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0
arduino:mbed:envie_m7,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0
arduino:mbed:nano33ble,0,0.0,0,0.0,,,,,,,,,,,,,,,,,,,,,,,,,0,0.0,0,0.0,,,,,,,,,,,,,0,0.0,0,0.0