tty: move tty_ldisc docs to new Documentation/tty/

Create a new directory in Documentation/ called tty. We will create more
documents in it in the next patches, so let's have this one in the very
same place.

Change title accordingly and all the headers. This is the way what other
documents look like in this directory in the next patches. So make this
unified.

And add a TOC.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211126081611.11001-20-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>