Bump sphinxcontrib-serializinghtml from 1.1.5 to 2.0.0 in /docs

Bumps [sphinxcontrib-serializinghtml](https://github.com/sphinx-doc/sphinxcontrib-serializinghtml) from 1.1.5 to 2.0.0.
- [Release notes](https://github.com/sphinx-doc/sphinxcontrib-serializinghtml/releases)
- [Changelog](https://github.com/sphinx-doc/sphinxcontrib-serializinghtml/blob/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinxcontrib-serializinghtml@1.1.5...2.0.0)

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

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

docs/requirements.txt

@@ -9,5 +9,5 @@ sphinx-copybutton==0.5.1
 sphinxcontrib-applehelp==1.0.3
 sphinxcontrib.devhelp==1.0.2
 sphinxcontrib-htmlhelp==2.0.0
-sphinxcontrib-serializinghtml==1.1.5
+sphinxcontrib-serializinghtml==2.0.0
 sphinxcontrib-qthelp==1.0.3