Bump lxml-html-clean from 0.1.1 to 0.4.0 in /docs

Bumps [lxml-html-clean](https://github.com/fedora-python/lxml_html_clean) from 0.1.1 to 0.4.0.
- [Changelog](https://github.com/fedora-python/lxml_html_clean/blob/main/CHANGES.rst)
- [Commits](fedora-python/lxml_html_clean@0.1.1...0.4.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

docs/requirements_docs.txt

@@ -11,5 +11,5 @@ sphinx-book-theme==1.1.2
 pandoc==2.3
 sphinx-rtd-theme==2.0.0
 readthedocs-sphinx-ext==2.2.5
-lxml-html-clean==0.1.1
+lxml-html-clean==0.4.0
 pydata-sphinx-theme==0.15.2