Bump actions/checkout from 5 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

.github/workflows/docs.yml

@@ -12,7 +12,7 @@ jobs:
       matrix:
         python-version: ['3.14']
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Filter changed file paths to outputs
         uses: dorny/paths-filter@v3.0.2

.github/workflows/tests.yml

@@ -12,7 +12,7 @@ jobs:
         python-version: ['3.10', '3.11', '3.12', '3.13', '3.14']
         docutils-version: ['0.18', '0.19']
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Install uv
         uses: astral-sh/setup-uv@v7
@@ -56,7 +56,7 @@ jobs:
         python-version: ['3.14']
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Install uv
         uses: astral-sh/setup-uv@v7