Automate documentation deployment on GitHub

[glennmoy]

When we create a new package with PkgTemplates.jl it does almost all the work. The one manual step is setting up documentation deployment on GitHub (it is automatic on GitLab). Which is an annoying process that involves manually copy-pasting around some private/public keys, and folks often screw it up by not copying all the letters etc.

[nickrobinson251]

Do you know what additional steps are required? And do you know which could possibly be automated? (for ones that PkgTemplates.jl can't/won't automate, we could still provide some kind of instructions on next steps; xref https://github.com/invenia/PkgTemplates.jl/issues/318)