Update documentation that refers to vestigial /etc/qubes/guid.conf, and eventually remove it

[Euwiiwueir]

How to file a helpful issue

Qubes OS release

4.1+

Brief summary

Global and per-VM qubes-guid configuration used to be done by way of the file /etc/qubes/guid.conf. Since release 4.1 configuration of qubes-guid transitioned to qvm-features, with this updated note in the manpage:

   gui-*, gui-default-*
       GUI  daemon  configuration. See /etc/qubes/guid.conf for a list of sup‐
       ported options.

       To change a given GUI option for a specific qube, set the  gui-{option}
       feature (with underscores replaced with dashes). For example, to enable
       allow_utf8_titles for a qube, set gui-allow-utf8-titles to True.

       To change a given GUI option  globally,  set  the  gui-default-{option}
       feature on the GuiVM for that qube.

And in /etc/qubes/guid.conf:

# Default configuration file for Qubes GUI daemon
# (For syntax go http://www.hyperrealm.com/libconfig/libconfig_manual.html)

# EDITING THIS FILE WILL NOT HAVE ANY EFFECT! It is not read by the GUI daemon
# anymore, and is left here for reference only. To configure the GUI daemon,
# use qvm-features (see 'man qvm-features' for details).

...

However, other documentation refers to this file as still being effective:

https://www.qubes-os.org/doc/config-files/#gui-and-audio-configuration-in-dom0
https://github.com/QubesOS/qubes-doc/blob/03ebdad8f96e0f21f65a5e1f1f9ade5af9b254c9/user/advanced-topics/config-files.md

https://www.qubes-os.org/doc/how-to-enter-fullscreen-mode/#enabling-fullscreen-mode-for-select-vms
https://github.com/QubesOS/qubes-doc/blob/03ebdad8f96e0f21f65a5e1f1f9ade5af9b254c9/user/how-to-guides/how-to-enter-fullscreen-mode.md

To fix:

• Move the useful documentation remaining in /etc/qubes/guid.conf directly into the qvm-features manpage (https://github.com/QubesOS/qubes-core-admin-client/blob/main/doc/manpages/qvm-features.rst)
• Update https://github.com/QubesOS/qubes-doc/blob/main/user/how-to-guides/how-to-enter-fullscreen-mode.md
• Update https://github.com/QubesOS/qubes-doc/blob/main/user/advanced-topics/config-files.md
• Update xside.c, xside.h to remove references to /etc/qubes/guid.conf
• Eventually: remove /etc/qubes/guid.conf and connected build/installation infrastructure

Steps to reproduce

Click documentation links above

Expected behavior

Documentation refers to qvm-features rather than /etc/qubes/guid.conf

Actual behavior

Documentation is misleading