Bump sphinxcontrib-phpdomain from 0.2.1 to 0.12.0 in /docs

Bumps [sphinxcontrib-phpdomain](https://github.com/markstory/sphinxcontrib-phpdomain) from 0.2.1 to 0.12.0.
- [Release notes](https://github.com/markstory/sphinxcontrib-phpdomain/releases)
- [Changelog](https://github.com/markstory/sphinxcontrib-phpdomain/blob/master/CHANGES)
- [Commits](markstory/sphinxcontrib-phpdomain@0.2.1...0.12.0)

---
updated-dependencies:
- dependency-name: sphinxcontrib-phpdomain
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

docs/requirements.txt

@@ -14,6 +14,6 @@ six==1.11.0
 snowballstemmer==1.2.1
 Sphinx==1.5.1
 sphinx-rtd-theme==0.1.9
-sphinxcontrib-phpdomain==0.2.1
+sphinxcontrib-phpdomain==0.12.0
 urllib3==1.26.19
 cakephp-theme