[Unified Doc Viewer] Exclude static style assets from Unified Doc Viewer tree shaking (#167199)

## Summary

This PR updates tree shaking in Unified Doc Viewer to exclude static
style assets.

Before:

![image](5e07b4e3-ae73-4596-896e-f8f4900736c3)

After:

![image](ac13a4f8-b680-4d70-bf73-85bd02294711)

Test by launching Kibana before and after the fix with prod bundles and
no bundle caching:
```
node scripts/kibana --dev --dist --no-cache
```

Fixes #167198.

### Checklist

- [ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/packages/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [ ] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [ ] Any UI touched in this PR is usable by keyboard only (learn more
about [keyboard accessibility](https://webaim.org/techniques/keyboard/))
- [ ] Any UI touched in this PR does not create any new axe failures
(run axe in browser:
[FF](https://addons.mozilla.org/en-US/firefox/addon/axe-devtools/),
[Chrome](https://chrome.google.com/webstore/detail/axe-web-accessibility-tes/lhdoppojpmngadmnindnejefpokejbdd?hl=en-US))
- [ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [ ] This renders correctly on smaller devices using a responsive
layout. (You can test this [in your
browser](https://www.browserstack.com/guide/responsive-testing-on-local-server))
- [ ] This was checked for [cross-browser
compatibility](https://www.elastic.co/support/matrix#matrix_browsers)

### For maintainers

- [ ] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

packages/kbn-unified-doc-viewer/package.json

@@ -3,5 +3,8 @@
   "private": true,
   "version": "1.0.0",
   "license": "SSPL-1.0 OR Elastic License 2.0",
-  "sideEffects": false
-}
+  "sideEffects": [
+    "*.css",
+    "*.scss"
+  ]
+}