add netlify previews for PRs?

besides using git-annex to manage images etc content I do not think there are limiting factors forbidding generating previews for PRs, in particular -- they could potentially limit only to the content necessary!

ATM annex info say

```
size of annexed files in working tree: 135.93 megabytes
```
so we might potentially start hitting limits on github gh-pages (what are they?)

### Optimization

1. we could just `get` only the talks/slides touched by PR
2. we could potentially fetch only the content needed for those particular slides!

We used to have a 'tracking' functionality in datalad to monitor for open files and auto 'annex get' it so then we could have `-m http.server` and get those . That is gone. But I think it might be feasible to make it work with datalad-fuse if we add mode to simply `annex get` all files instead of trying to proxy via fsspec etc. Should be the easiest! then we would publish result after going through all the slides in the talk