LP1930099: Make generate_docs.pl Windows-compatible

LP1930099: Make generate_docs.pl Windows-compatible

* Use `npx` rather than relying on specific paths to
binaries within node_modules.
* Use the cross-platform $ENV and File::Copy provided
by Perl, rather than using the platform-specific
command-line syntax.

To test on Windows:
1) Use the instructions provided in the comment at
the top of generate_docs.pl.
2) Make sure that the locally generated documentation
displays correctly in your browser.

To test for regressions on Linux or MacOs:
1) `cd /path/to/Evergreen/docs && perl generate_docs.pl`
2) Make sure that the locally generated documentation
at /path/to/Evergreen/docs/output/index.html displays
correctly in your browser.

Release-note: Allow Windows users to generate the Evergreen manual locally.

Signed-off-by: Jane Sandberg <sandbergja@gmail.com>
Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>
Signed-off-by: Andrea Buntz Neiman <abneiman@equinoxOLI.org>