chore(deps): bump pymdown-extensions from 10.16.1 to 10.21.3 in /docs

Bumps [pymdown-extensions](https://github.com/facelessuser/pymdown-extensions) from 10.16.1 to 10.21.3.
- [Release notes](https://github.com/facelessuser/pymdown-extensions/releases)
- [Commits](facelessuser/pymdown-extensions@10.16.1...10.21.3)

---
updated-dependencies:
- dependency-name: pymdown-extensions
  dependency-version: 10.21.3
  dependency-type: indirect
...

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

Author: dependabot[bot]

docs/requirements.txt

@@ -324,9 +324,9 @@ pygments==2.19.2 \
     --hash=sha256:636cb2477cec7f8952536970bc533bc43743542f70392ae026374600add5b887 \
     --hash=sha256:86540386c03d588bb81d44bc3928634ff26449851e99741617ecb9037ee5ec0b
     # via mkdocs-material
-pymdown-extensions==10.16.1 \
-    --hash=sha256:aace82bcccba3efc03e25d584e6a22d27a8e17caa3f4dd9f207e49b787aa9a91 \
-    --hash=sha256:d6ba157a6c03146a7fb122b2b9a121300056384eafeec9c9f9e584adfdb2a32d
+pymdown-extensions==10.21.3 \
+    --hash=sha256:72cfcf55f07aea0d4af2c4f11dd4e52466ddfb1bb819673146398e0bd3a77354 \
+    --hash=sha256:d7a5d08014fc571e80ca21dd6f854e31f94c489800350564d55d15b3c41e76b6
     # via
     #   mkdocs-material
     #   mkdocstrings