Documentation: move tty to driver-api

Based on discussion starting as [e-mail], let's move
the tty documentation to driver-api. It's more appropriate there.

Cc: Jonathan Corbet <[e-mail]>
Signed-off-by: Jiri Slaby <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/driver-api/index.rst

···
    surface_aggregator/index
    switchtec
    sync_file
+   tty
    vfio-mediated-device
    vfio
    vfio-pci-device-specific-driver-acceptance

Documentation/driver-api/serial/driver.rst

···
 	This call must not sleep
 
   set_ldisc(port,termios)
-	Notifier for discipline change. See Documentation/tty/tty_ldisc.rst.
+	Notifier for discipline change. See ../tty/tty_ldisc.rst.
 
 	Locking: caller holds tty_port->mutex
 

Documentation/index.rst

···
    misc-devices/index
    scheduler/index
    mhi/index
-   tty/index
    peci/index
 
 Architecture-agnostic documentation

Documentation/tty/index.rst Documentation/driver-api/tty/index.rst

···
 ==================
 
 Before one starts writing a TTY driver, they must consider
-:doc:`Serial <../driver-api/serial/driver>` and :doc:`USB Serial
-<../usb/usb-serial>` layers
-first. Drivers for serial devices can often use one of these specific layers to
-implement a serial driver. Only special devices should be handled directly by
-the TTY Layer. If you are about to write such a driver, read on.
+:doc:`Serial <../serial/driver>` and :doc:`USB Serial <../../usb/usb-serial>`
+layers first. Drivers for serial devices can often use one of these specific
+layers to implement a serial driver. Only special devices should be handled
+directly by the TTY Layer. If you are about to write such a driver, read on.
 
 A *typical* sequence a TTY driver performs is as follows: