Bump idna from 3.11 to 3.15 in /docs

Bumps [idna](https://github.com/kjd/idna) from 3.11 to 3.15.
- [Release notes](https://github.com/kjd/idna/releases)
- [Changelog](https://github.com/kjd/idna/blob/master/HISTORY.md)
- [Commits](kjd/idna@v3.11...v3.15)

---
updated-dependencies:
- dependency-name: idna
  dependency-version: '3.15'
  dependency-type: direct:production
...

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

Author: dependabot[bot]

docs/requirements.txt

@@ -6,7 +6,7 @@ certifi==2026.1.4
 charset-normalizer==3.4.4
 docutils==0.22.4
 furo==2025.12.19
-idna==3.11
+idna==3.15
 imagesize==1.4.1
 Jinja2==3.1.6
 MarkupSafe==3.0.3