Recognise a site repo by templates/include

The sanity check looked for a top-level include/ directory. That directory
exists in a local checkout but is empty, and git does not track empty
directories, so it is absent after a fresh clone and the build refused to run
in CI:

    ERROR: '...' does not look like a site repo (missing include/)

Nothing reads it. Templates referring to include/intro.html resolve through
the lookup rule that retries under templates/, where the two files actually
live. Check for templates/include instead.

Author: z3APA3A

tools/buildsite.py

@@ -425,8 +425,8 @@ def main():
 
     for site_arg in args:
         site_dir = os.path.abspath(site_arg)
-        if not os.path.isdir(os.path.join(site_dir, 'include')):
-            print(f"ERROR: '{site_dir}' does not look like a site repo (missing include/)", file=sys.stderr)
+        if not os.path.isdir(os.path.join(site_dir, 'templates', 'include')):
+            print(f"ERROR: '{site_dir}' does not look like a site repo (missing templates/include/)", file=sys.stderr)
             sys.exit(1)
         build_site(site_dir, content_dir)