build napari-docs on theme PR

[psobolewskiPhD]

Add a GitHub workflow to build the current napari docs (dev) using a PR branch of this theme.
Note at the moment I'm letting the gallery build, but deleting the .doctrees folder which was 400Mb!

This partially addresses #155
The CircleCI previews would be more user friendly, but require some extra secrets.

[psobolewskiPhD]

You can see for example the mastodon link from previous PR:
#153

[psobolewskiPhD]

Dang, nuking the .doctrees saved 500Mb. the artifacts listed as <100 mb and unzipped it's <130 locally:

❯ du -h -d 1
832K	./_downloads
296K	./roadmaps
5.4M	./_modules
2.1M	./plugins
7.2M	./gallery
2.5M	./_sources
 18M	./_static
236K	./further-resources
3.2M	./release
 63M	./_images
724K	./_tags
1.3M	./guides
6.1M	./api
4.6M	./tutorials
 52K	./_sphinx_design_static
600K	./community
8.4M	./howtos
1.1M	./naps
1.9M	./developers
129M	.

[psobolewskiPhD]

As an alternative we could build just the mini-site that is in this repo, but it has few of the napari.org features.
You can see that here:
psobolewskiPhD#1

[jni]

Fantastic, thanks! 🚀