Gallery: make prettier thumbnails

The thumbnails in the gallery are mostly generated on the fly by `nbsite`. While this is useful to make sure they are in sync with the notebook output and it's somewhat easier to maintain, this has a few drawbacks:

- thumbnails should be crafted with care to make the gallery prettier (e.g. they should be squarish)
- it makes the docs build slower
- it makes the docs build more difficult to set up as you need some browser extension to save Bokeh figures

So let's make some nice thumbnails. When they're built, they'll need to be hosted on S3 as done by e.g. Panel for `nbsite` to fetch them when it builds the site.

```python
nbsite_gallery_conf = {
....
'thumbnail_url': 'https://assets.holoviews.org/panel/thumbnails',
...
}
```