Fix examples path in build_docs PR workflow

[psobolewskiPhD]

Description

The build action appears to be failing, see https://github.com/napari/docs/actions/runs/3254781307 with an error related to the path of the examples:

Extension error:
[52](https://github.com/napari/docs/actions/runs/3254781307/jobs/5343418375#step:9:53)
Example directory /home/runner/work/docs/docs/docs/docs/../../napari/examples does not have a README file with one of the expected file extensions ['.txt', '.md', '.rst']. Please write one to introduce your gallery.
[53](https://github.com/napari/docs/actions/runs/3254781307/jobs/5343418375#step:9:54)
make: *** [Makefile:15: docs-build] Error 2

I try to fix the nesting, because it is tricky: the docs building on napari/napari (also failing at the moment see: https://github.com/napari/napari/actions/runs/3254935774) use this same conf.py. Hence doing the same copy step to ensure the paths are correct. This will fix the failing napari workflow too, because now conf.py should be correct—tested with my forks:
Here's the docs fork, showing the workflow works:
https://github.com/psobolewskiPhD/napari-docs/actions/runs/3255346428
and here's my napari fork, showing that workflow now works with no changes:
https://github.com/psobolewskiPhD/napari/actions/runs/3255523293

Type of change

• Bug-fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

References

How has this been tested?

• example: the test suite for my feature covers cases x, y, and z
• example: all tests pass with my change

Final checklist:

• My PR is the minimum possible work for the desired functionality
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• If I included new strings, I have used trans. to make them localizable.
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[jni]

I think indeed that this is nicer! 🙏