Documentation: Deprecated signing key import method

[daira]

Split from #6742 by @metalworker2 :

---

I found these issues in the docs when I was following the instructions for installing on Debian/Ubuntu officially supported platform:

https://zcash.readthedocs.io/en/latest/rtd_pages/install_debian_bin_packages.html

Deprecated signing key import method

When I run the suggested command:

wget -qO - https://apt.z.cash/zcash.asc | gpg --import
gpg --export B1C9095EAA1848DBB54D9DDA1D05FDC66B372CFE | sudo apt-key add -

The terminal says this message:

Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)).

It worked anyways, as far as I can tell. But since the terminal gives this warning and deprecation notice, maybe better to use the modern command for this.