Bump pygments from 2.10.0 to 2.15.0 in /docs

Bumps [pygments](https://github.com/pygments/pygments) from 2.10.0 to 2.15.0.
- [Release notes](https://github.com/pygments/pygments/releases)
- [Changelog](https://github.com/pygments/pygments/blob/master/CHANGES)
- [Commits](pygments/pygments@2.10.0...2.15.0)

---
updated-dependencies:
- dependency-name: pygments
  dependency-type: direct:production
...

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

docs/rtd-requirements.txt

@@ -18,7 +18,7 @@ pexpect==4.8.0
 pickleshare==0.7.5
 prompt-toolkit==3.0.20
 ptyprocess==0.7.0
-Pygments==2.10.0
+Pygments==2.15.0
 pyparsing==2.4.7
 pytz==2021.1
 pyzmq==22.3.0