Better documentation of LEKTOR_DEPLOY_KEY

[nixjdm]

LEKTOR_DEPLOY_KEY and LEKTOR_DEPLOY_KEY_FILE can be used to supply credentials for both rsync and GitHub pages. Only the rsync page mentions this at all, and doesn't explain well-enough what's going on. I think the docs are kind of assuming it's obvious that these are environment variables that are looked for.

The GitHub Pages page doesn't mention them at all, but reading through publishers.py, it can clearly be used there too.