Installation instructions: context for use of snap --classic

[bushjam1]

In the installation instructions guide for installing Certbot on, e.g., Ubuntu, Sections 2 and 5 of the instructions reference using snap to install Certbot with the --classic flag. For first-time or infrequent users of snap, some context on use of the flag might be helpful, much like the reference made in Section to on installation of snapd. This could take the form of an edit to existing along the lines of:

"5. Install Certbot

Run this command on the command line on the machine to install Certbot. Running snap with the --classic flag allows limited access to system resources necessary for Certbot to function. See the Snapcraft site for details on the use and purpose of classic confinement for snap.”

[bmw]

Hmm. I think we should consider doing this if we get more requests for it.

Rather than increasing the verbosity of the instructions, my initial thought is on the first reference of classic snaps, make the text a link to https://snapcraft.io/docs/snap-confinement.