tool/.gitattributes: exclude doc/html/.doctrees/ from zip exports

- https://www.sphinx-doc.org/en/master/man/sphinx-build.html#cmdoption-sphinx-build-d
- https://stackoverflow.com/questions/33904042/is-doctrees-folder-required-for-displaying-html-docs-with-sphinx

.gitattributes

@@ -42,5 +42,6 @@ Makefile          export-ignore
 node_modules/     export-ignore
 doc/conf.py       export-ignore
 doc/requirements.txt export-ignore
+doc/html/.doctrees/  export-ignore
 phpunit.xml       export-ignore
 tests/            export-ignore