Add list of settings

[SteveDesmond-ca]

As a new Statiq user, I'd like a one-stop shop to know what options I can add to my appsettings.json file, so that I can most efficiently configure my site.

I was able to figure out what I needed by browsing the framework code, but it'd be great if the full list was laid out and described somewhere handy!

[daveaglick]

So much this. There are pages with many of them: https://www.statiq.dev/web/configuration/settings and (Statiq Web) and https://www.statiq.dev/framework/configuration/settings (Statiq Framework) - but my longer-term plan is to consolidate that into a single auto-generated page that has all settings and indicates where you can use them. Hopefully I can light something like that up as soon as Statiq Docs ships (dogfooding and all that).