[FIX] website: unpublished homepage read error + first menu fallback

Before this commit:
1. If the website's homepage is unpublished and a public user naviguate on it,
it will log an access read error in the server. The user won't see the error
though.

2. If the user can't access '/', we serve the first menu's page instead.
This first menu's page can be an unpublished page, resulting in a 403, which is
not convenient.

3. Menus are sorted by sequence, then the ORM sort them by write_date.
Menu with same sequence will be sorted differently every time you edit one of
them. (This issue get hidden/solved if user edit and save the menus with
"Edit Menu" since it will recompute every sequence.)
  - Create 3 pages 'A', 'B', 'C', with 'Add page in menu' enabled
  - Menu is now 'A B C'
  - Edit website.page B (in property dialog), it will write on the page's menu
    aswell
  - Menu is now 'A C B'

Now:
1. We prevent this error by ensuring the page is published and so can be read
by anyone before accessing its properties.

2. We serve the first menu's page that is published

3. We sort menu by id after sequence to keep original order

This closes #21259

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