migrate ads to (remote) service instead of git-submodule

[gernotstarke]

Basic idea:

[ ] deploy subtle-ads service (API) that delivers these ads as HTML-div, either in Go or Python
[ ] this div shall be included in docs, faq (and likely others) by means of htmx (https://htmx.org), similar to site-usage-statistics
with a delay of about 1500-2000msec
[ ] deploy this service with fly.io (or vercel)
[ ] caution: CORS issues may arise - these have been solved in site-usage-statistics (in golang)

Advantage:

1. We don't need to mess around with submodules and their dependency on branch names
2. Google and the Google-search console hopefully won't recognize the (delayed) text as duplicate, improving searchability
3. it's cooler