Bump sphinxcontrib-spelling from 5.4.0 to 6.0.0 (#538)

Bumps [sphinxcontrib-spelling](https://github.com/sphinx-contrib/spelling) from 5.4.0 to 6.0.0.
- [Release notes](https://github.com/sphinx-contrib/spelling/releases)
- [Commits](sphinx-contrib/spelling@5.4.0...6.0.0)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements/doc-spelling.txt

@@ -1,2 +1,2 @@
 -r doc.txt
-sphinxcontrib-spelling==5.4.0; platform_system!="Windows"  # We only use it in Azure CI
+sphinxcontrib-spelling==6.0.0; platform_system!="Windows"  # We only use it in Azure CI