Added a new tutorial for connecting to a serial port through a usb-to-uart converter, as is the cases with most SoC's

[arshankhanifar_gmail.com]

changed the diff to one with context

[arshankhanifar_gmail.com]

Applied the changes suggested by @bcr.
Also, expanded the explanation with a brief tutorial on how to communicate with the device through the cu command.

[bcr]

A few more suggestions.

book.withusb.xml

6359

You should mark this up with replaceable tags around the X, so that it is clear that this is the part the user has to change:
<filename>ttyU<replaceable>X</replaceable></filename>

6360

the number of the device.

6362

It's simpler to write:
Use <command>cu</command> to establish a connection through USB to UART.

6367

I would also markup speed in <replaceable> tags here. Examples should be something a user could enter at the prompt and it would work, even though the speed is not the correct one. Having replaceable tags, you can reference it in the sentence following the example to change the speed to the one fitting the device.
For example, you can write: The following example connects to a Beaglebone Black using a baud-rate of 115,200: (example follows with that number in the example, surrounded by <replaceable> tags) Next paragraph: For other devices, replace the baud-rate with the one typically found on the device vendor documentation or website.

[arshankhanifar_gmail.com]

applied the changes as suggested by @bcr

book.withusb.xml

6367

done

[bcr]

One small nit.

book.withusb.new.xml

6363 ↗

(On Diff #38322)

Needs a space between "Use" and <command>.

[arshankhanifar_gmail.com]

book.withusb.new.xml

6363 ↗

(On Diff #38322)

fixed

[arshankhanifar_gmail.com]

tiny space between cu and command