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

@@ -24,7 +24,7 @@ et-xmlfile==2.0.0
     # via openpyxl
 fonttools==4.61.0
     # via matplotlib
-idna==3.11
+idna==3.15
     # via requests
 imagesize==1.4.1
     # via sphinx