docs: add standalone Contributor License Agreement page (refs #12863)

Issue #12863 reports that the Contributor License Agreement is rendered
within docs/CONTRIBUTING along with unrelated content, making it hard to
read in isolation. The repository did not previously expose a dedicated
CLA page.

Add docs/CONTRIBUTOR_LICENSE_AGREEMENT.md containing only the CLA text
(rights, representations, license grant), include it in _config.yml, and
link to it from the CLA section of CONTRIBUTING. Users who want to read
only the CLA now have a clean, focused page.

_config.yml

@@ -17,6 +17,7 @@ relative_links:
 
 include:
   - CONTRIBUTING.md
+  - CONTRIBUTOR_LICENSE_AGREEMENT.md
   - LICENSE.md
   - CODE_OF_CONDUCT.md
 

docs/CONTRIBUTING.md

@@ -4,6 +4,8 @@
 ## Contributor License Agreement
 
 By contributing, you agree to the [LICENSE](../LICENSE) of this repository.
+See the full [Contributor License Agreement](CONTRIBUTOR_LICENSE_AGREEMENT.md)
+for details.
 
 
 ## Contributor Code of Conduct

docs/CONTRIBUTOR_LICENSE_AGREEMENT.md

@@ -0,0 +1,23 @@
+---
+title: Contributor License Agreement
+---
+
+# Contributor License Agreement
+
+By submitting a contribution (code, content, translations, or any other material)
+to this repository, you agree that your contribution is licensed under the terms
+of the project [LICENSE](../LICENSE).
+
+You also confirm that:
+
+- You have the right to submit the work under that license.
+- The work is your original creation, or you have obtained appropriate permission
+  to submit it.
+- You grant the project maintainers the right to use, modify, and distribute your
+  contribution as part of this repository.
+
+If you do not agree with these terms, please do not submit a contribution.
+
+For a full description of the contribution workflow and guidelines, see
+[CONTRIBUTING](CONTRIBUTING.md). For the Code of Conduct, see
+[CODE_OF_CONDUCT](CODE_OF_CONDUCT.md).