Bump lxml-html-clean from 0.2.2 to 0.4.0 in /docs in the pip group

Bumps the pip group in /docs with 1 update: [lxml-html-clean](https://github.com/fedora-python/lxml_html_clean).


Updates `lxml-html-clean` from 0.2.2 to 0.4.0
- [Changelog](https://github.com/fedora-python/lxml_html_clean/blob/main/CHANGES.rst)
- [Commits](fedora-python/lxml_html_clean@0.2.2...0.4.0)

---
updated-dependencies:
- dependency-name: lxml-html-clean
  dependency-type: direct:production
  dependency-group: pip
...

Signed-off-by: dependabot[bot] <[email protected]>

docs/requirements.txt

@@ -7,4 +7,4 @@ nbsphinx==0.9.3
 sphinx==7.1.2
 nbconvert==6.5.4
 lxml==5.2.1
-lxml_html_clean==0.2.2
+lxml_html_clean==0.4.0