feat: add xp-clifford submodule with sidebar and config integration

Add xp-clifford as a git submodule under docs/ and integrate it into
the Docusaurus site:

- Register submodule pointing to SAP/xp-clifford.git
- Add exclude rules in docusaurus.config.js to filter out non-docs
  content (subdirectories, CONTRIBUTING.md, DEVELOPMENT.md)
- Add xp-clifford entry to the navbar dropdown menu
- Add collapsed sidebar category with autogenerated items in sidebars.js

Author: gergely-szabo-sap

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "docs/crossplane-provider-hana"]
 	path = docs/crossplane-provider-hana
 	url = https://github.com/SAP/crossplane-provider-hana.git
+[submodule "docs/xp-clifford"]
+	path = docs/xp-clifford
+	url = git@github.com:SAP/xp-clifford.git

docs/xp-clifford

@@ -45,6 +45,11 @@ const config = {
             'crossplane-provider-btp/!(docs)',
             'crossplane-provider-hana/!(docs)/**',
             'crossplane-provider-hana/!(docs)',
+            // xp-clifford: docs live in README.md at the repo root, not in a docs/ subdirectory.
+            // Exclude all subdirectories and non-documentation .md files.
+            'xp-clifford/*/**',
+            'xp-clifford/CONTRIBUTING.md',
+            'xp-clifford/DEVELOPMENT.md',
           ],
         },
         blog: false,
@@ -96,6 +101,10 @@ const config = {
                 label: 'crossplane-provider-hana',
                 to: '/docs/crossplane-provider-hana/docs/ADR/20250822-hana-x509-authentication',
               },
+              {
+                label: 'xp-clifford',
+                to: '/docs/xp-clifford',
+              },
             ],
           },
           {

docusaurus.config.js

@@ -39,6 +39,18 @@ const sidebars = {
         },
       ],
     },
+    {
+      type: 'category',
+      label: 'xp-clifford',
+      collapsible: true,
+      collapsed: true,
+      items: [
+        {
+          type: 'autogenerated',
+          dirName: 'xp-clifford',
+        },
+      ],
+    },
     // To add a new submodule, copy the block above and adjust label and dirName, e.g.:
     // {
     //   type: 'category',