build(deps): bump sphinxcontrib-serializinghtml in /docs

Bumps [sphinxcontrib-serializinghtml](https://github.com/sphinx-doc/sphinxcontrib-serializinghtml) from 1.1.5 to 1.1.9.
- [Release notes](https://github.com/sphinx-doc/sphinxcontrib-serializinghtml/releases)
- [Changelog](https://github.com/sphinx-doc/sphinxcontrib-serializinghtml/blob/master/CHANGES)
- [Commits](sphinx-doc/sphinxcontrib-serializinghtml@1.1.5...1.1.9)

---
updated-dependencies:
- dependency-name: sphinxcontrib-serializinghtml
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

docs/requirements.txt

@@ -20,7 +20,7 @@ sphinx-favicon==1.0.1
 sphinx-rtd-theme==1.2.2
 sphinxcontrib-applehelp==1.0.4
 sphinxcontrib-devhelp==1.0.2
-sphinxcontrib-serializinghtml==1.1.5
+sphinxcontrib-serializinghtml==1.1.9
 sphinxcontrib-htmlhelp==2.0.1
 sphinxcontrib-jsmath==1.0.1
 sphinxcontrib-qthelp==1.0.3