add missing site_url and hide some warnings

mkdocs.yaml

@@ -1,5 +1,6 @@
 # https://www.mkdocs.org/user-guide/configuration/
 site_name: RSE@PSI
+site_url: https://rse.psi.ch
 copyright: © Paul Scherrer Institute (PSI) | Content under CC0 License
 nav:
   - Home: 'index.md'
@@ -11,6 +12,9 @@ nav:
     - Events: 'events.md'
     - Surveys: 'surveys.md'
   - Blog: 'blog/index.md'
+not_in_nav: |
+  # don't issue a warning about missing files in nav
+  events/**
 theme: # https://www.mkdocs.org/user-guide/choosing-your-theme/#readthedocs
   name: material
   custom_dir: docs/_overrides