Reduce boilerplate for setting up new pages

[richard-to]

Currently we need to make updates in two places:

side_nav.py -> to add the menu entry
main.py -> mostly boilerplate to add the page.

A lot of this is copy and paste and then make some edits.

@me.page(
  path="/",
  title="Mesop / Gemini Experiments",
  security_policy=me.SecurityPolicy(
    allowed_script_srcs=[
      "https://cdn.jsdelivr.net",
    ]
  ),
  on_load=on_load,
)
def home_demo():
  """Main Page"""
  state = me.state(AppState)
  with page_scaffold():  # pylint: disable=not-context-manager
    home_content(state)

I don't have any specific suggestions on how to reduce the boilerplate though. I think fine for now. Can always ask an LLM to write the boilerplate anyway.