Include list of third-party licenses in the C++ docs

This isn't very pretty, but I think it does the job.

cc #6634

api/cpp/docs/index.rst

@@ -47,6 +47,13 @@ Slint C++ documentation
 
    genindex
 
+.. toctree::
+   :maxdepth: 0
+   :hidden:
+   :caption: Third-Party Licenses
+
+   thirdparty.md
+
 .. image:: https://github.com/slint-ui/slint/workflows/CI/badge.svg
    :target: https://github.com/slint-ui/slint/actions
    :alt: GitHub CI Build Status

api/cpp/docs/thirdparty.hbs

@@ -0,0 +1,29 @@
+# Third Party Licenses
+
+This page lists the licenses of the dependencies used by Slint.
+
+## Overview of licenses
+
+{{#each overview}}
+- [{{name}}](#{{id}}) ({{count}})
+{{/each}}
+
+## All License Text
+
+{{#each licenses}}
+### <a id="{{id}}"></a> {{name}}
+
+#### Used by:
+
+{{#each used_by}}
+- [{{crate.name}} {{crate.version}}]({{#if crate.repository}} {{crate.repository}} {{else}}
+https://crates.io/crates/{{crate.name}} {{/if}})
+{{/each}}
+
+#### License Text
+
+```
+{{text}}
+```
+
+{{/each}}