docs/install.rst: do GPG key setup before setting sources for debian

[baprusty]

Fixes GPG warning observed while performing 'apt update' operation:
GPG error: https://apt.fury.io/mtda InRelease: The following signatures couldn't be verified because the public key is not available

Signed-off-by: Badrikesh Prusty [email protected]

Apt warning observed:

badrikesh@debian:~$ sudo apt update
Hit:1 http://deb.debian.org/debian bookworm InRelease
Hit:2 http://security.debian.org/debian-security bookworm-security InRelease
Hit:3 http://deb.debian.org/debian bookworm-updates InRelease
Hit:4 http://deb.debian.org/debian bookworm-backports InRelease
Get:5 https://apt.fury.io/mtda  InRelease [1,975 B]
Ign:5 https://apt.fury.io/mtda  InRelease
Get:6 https://apt.fury.io/mtda  Packages [228 kB]
Fetched 230 kB in 3s (87.6 kB/s)
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
2 packages can be upgraded. Run 'apt list --upgradable' to see them.
W: GPG error: https://apt.fury.io/mtda  InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY C8F9D89C247C0257

After adding gpg key:

badrikesh@debian:~$ ls -l /etc/apt/keyrings
total 4
-rw-r--r-- 1 root root 2265 Oct 23 03:45 mtda.gpg
badrikesh@debian:~$ cat /etc/apt/sources.list.d/mtda.list
deb [arch=amd64 signed-by=/etc/apt/keyrings/mtda.gpg] https://apt.fury.io/mtda/ /

badrikesh@debian:~$ sudo apt update
Hit:1 http://deb.debian.org/debian bookworm InRelease
Hit:2 http://security.debian.org/debian-security bookworm-security InRelease
Hit:3 http://deb.debian.org/debian bookworm-updates InRelease
Hit:4 http://deb.debian.org/debian bookworm-backports InRelease
Get:5 https://apt.fury.io/mtda  InRelease [1,975 B]
Fetched 1,975 B in 1s (1,446 B/s)
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
2 packages can be upgraded. Run 'apt list --upgradable' to see them.
badrikesh@debian:~$