Better manage site state so GHA deploys it

[arthur-shaw]

While adodownr has been updated to have better defaults and handle issues, the GHA needs to be updated too, in a few ways:

1. Ensure adodownr's new default not to delete site is explictly used
2. Have GitHub Pages Deploy action prune files that no longer exist on the site

In this PR, make edits to:

• adodown's own GHA job (i.e., the one that could deploy its adodown(r) site).
• adowdown's template that it instantiates for new pages

[arthur-shaw]

@kbjarkefur , how does this look to you?

See a few runs on test_gha that show that this works with the current YAML, and doesn't work if adodownr deletes the site before the deploy step