Bump idna from 3.13 to 3.15 in /llvm/docs

Bumps [idna](https://github.com/kjd/idna) from 3.13 to 3.15.
- [Release notes](https://github.com/kjd/idna/releases)
- [Changelog](https://github.com/kjd/idna/blob/master/HISTORY.md)
- [Commits](kjd/idna@v3.13...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]

llvm/docs/requirements-hashed.txt

@@ -167,9 +167,9 @@ furo==2025.12.19 \
     --hash=sha256:188d1f942037d8b37cd3985b955839fea62baa1730087dc29d157677c857e2a7 \
     --hash=sha256:bb0ead5309f9500130665a26bee87693c41ce4dbdff864dbfb6b0dae4673d24f
     # via -r requirements.txt
-idna==3.13 \
-    --hash=sha256:585ea8fe5d69b9181ec1afba340451fba6ba764af97026f92a91d4eef164a242 \
-    --hash=sha256:892ea0cde124a99ce773decba204c5552b69c3c67ffd5f232eb7696135bc8bb3
+idna==3.15 \
+    --hash=sha256:048adeaf8c2d788c40fee287673ccaa74c24ffd8dcf09ffa555a2fbb59f10ac8 \
+    --hash=sha256:ca962446ea538f7092a95e057da437618e886f4d349216d2b1e294abfdb65fdc
     # via requests
 imagesize==2.0.0 \
     --hash=sha256:5667c5bbb57ab3f1fa4bc366f4fbc971db3d5ed011fd2715fd8001f782718d96 \