Use napari_scraper instead of qtgallery (napari#207)

# Description
This is a CI change that removes `qtgallery` in favor of a
napari-specific scraper. It's not that different but gives us a little
more control and is not much code.

This seems to fix the error seen in "Build PR Docs" for
napari/napari#4865 and also speeds up the docs
build quite a bit (~11 min instead of ~25 min).

I'm no Qt expert but suspect the main improvements here are related to
adding `napari.Viewer.close_all()` (which maybe belongs in the reset fn)
and calling `processEvents()` one more time after this.

The main drawback right now is that this doesn't capture non-Viewer
windows, but this could probably be added if needed.

## Type of change
- [x] Fixes or improves workflow, documentation build or deployment

# References
closes napari#174 (maybe?)
fixes errors in docs build for
napari/napari#4865

## Final checklist:
- [x] My PR is the minimum possible work for the desired functionality
- [x] I have commented my code, particularly in hard-to-understand areas

Author: aganders3

docs/conf.py

@@ -168,12 +168,12 @@
 python_version_range = '3.8–3.10'
 
 myst_substitutions = {
-   "napari_conda_version": f"`napari={version_string}`",
-   "napari_version": version_string,
-   "python_version": python_version,
-   "python_version_range": python_version_range,
-   "python_version_code": f"`python={python_version}`",
-   "conda_create_env": f"```sh\nconda create -y -n napari-env -c conda-forge python={python_version}\nconda activate napari-env\n```",
+    "napari_conda_version": f"`napari={version_string}`",
+    "napari_version": version_string,
+    "python_version": python_version,
+    "python_version_range": python_version_range,
+    "python_version_code": f"`python={python_version}`",
+    "conda_create_env": f"```sh\nconda create -y -n napari-env -c conda-forge python={python_version}\nconda activate napari-env\n```",
 }
 
 myst_footnote_transition = False