Webcam (mjpeg-streamer) + gcodes dir documentation misalignement

[gcoppex]

Mjpg_streamer not configured

When installing Mainsail using the "Manual setup" on a Raspian 32 bits, there is no installation of mjpg_streamer. Hence the webcam support guide won't work.

(Side note, mjpeg-streamer documentation is slightly outdated and libjpeg9-dev needs to be installed instead of libjpeg8-dev)
What did you expect to happen instead?

Add a section in the ["Manual setup"](https://docs.mainsail.xyz/setup/manual-setup) about how to (compile and) install mjpg_streamer.
Add a note in the [Webcam guide page](https://docs.mainsail.xyz/features/webcams) that links to the aforementioned new section of the ["Manual setup"](https://docs.mainsail.xyz/setup/manual-setup)

Gcode file path / Virtual SD Card

The gcode path for the Virtual SD Card in the Required configuration seem to be outdated.

I experienced a "File not found" error, when printing a gcode.

According to rad126: the path should be replaced with the klipper instance name for each printer.

Now my printer.cfg reads:

[virtual_sdcard]
path: /home/pi/printer_data/gcodes