docs(r): fix source links from pkgdown site (#1434)

Same as apache/arrow-nanoarrow#315

Since the R package is in a subdirectory, I believe this setting is
necessary to ensure that the links to the source files are generated
correctly.

r/adbcdrivermanager/_pkgdown.yml

@@ -28,3 +28,10 @@ template:
           </a>
         </li>
       </ul>
+
+repo:
+  url:
+    home: https://github.com/apache/arrow-adbc/
+    source: https://github.com/apache/arrow-adbc/blob/main/r/adbcdrivermanager/
+    issue: https://github.com/apache/arrow-adbc/issues/
+    user: https://github.com/

r/adbcflightsql/_pkgdown.yml

@@ -28,3 +28,10 @@ template:
           </a>
         </li>
       </ul>
+
+repo:
+  url:
+    home: https://github.com/apache/arrow-adbc/
+    source: https://github.com/apache/arrow-adbc/blob/main/r/adbcflightsql/
+    issue: https://github.com/apache/arrow-adbc/issues/
+    user: https://github.com/

r/adbcpostgresql/_pkgdown.yml

@@ -28,3 +28,10 @@ template:
           </a>
         </li>
       </ul>
+
+repo:
+  url:
+    home: https://github.com/apache/arrow-adbc/
+    source: https://github.com/apache/arrow-adbc/blob/main/r/adbcpostgresql/
+    issue: https://github.com/apache/arrow-adbc/issues/
+    user: https://github.com/

r/adbcsnowflake/_pkgdown.yml

@@ -28,3 +28,10 @@ template:
           </a>
         </li>
       </ul>
+
+repo:
+  url:
+    home: https://github.com/apache/arrow-adbc/
+    source: https://github.com/apache/arrow-adbc/blob/main/r/adbcsnowflake/
+    issue: https://github.com/apache/arrow-adbc/issues/
+    user: https://github.com/

r/adbcsqlite/_pkgdown.yml

@@ -28,3 +28,10 @@ template:
           </a>
         </li>
       </ul>
+
+repo:
+  url:
+    home: https://github.com/apache/arrow-adbc/
+    source: https://github.com/apache/arrow-adbc/blob/main/r/adbcsqlite/
+    issue: https://github.com/apache/arrow-adbc/issues/
+    user: https://github.com/