[Docs] Update PPA installation: replace deprecated apt-key with gpg keyring

[TretiakovaN]

Hi! I wanted to submit a fix for the installation documentation, but direct Pull Requests seem to be restricted for non-collaborators (Validation failed: must be a collaborator).

The issue:
The current documentation uses apt-key, which is deprecated and not available in newer Debian/Ubuntu versions. Using it poses a security risk and triggers warnings.

The fix:
I updated the instructions to use the modern gpg --dearmor approach with signed-by in sources.list.

My commit with the fix:
8c1cc1e

---

Proposed installation block:

sudo apt install -y software-properties-common python3-launchpadlib gnupg2 linux-headers-$(uname -r)

# Import key using gpg
gpg --keyserver keyserver.ubuntu.com --recv-keys 57290828
gpg --export 57290828 | sudo gpg --dearmor -o /usr/share/keyrings/amnezia-archive-keyring.gpg

# Add repository with signed-by tag
echo "deb [signed-by=/usr/share/keyrings/amnezia-archive-keyring.gpg] https://ppa.launchpadcontent.net/amnezia/ppa/ubuntu focal main" | sudo tee -a /etc/apt/sources.list
echo "deb-src [signed-by=/usr/share/keyrings/amnezia-archive-keyring.gpg] https://ppa.launchpadcontent.net/amnezia/ppa/ubuntu focal main" | sudo tee -a /etc/apt/sources.list

sudo apt-get update
sudo apt-get install -y amneziawg