Improve the skeleton for shared hosting

I'd like to talk through ways that we can improve the experience for people on shared hosts.

References:
- https://github.com/flarum/docs/pull/42
- https://discuss.flarum.org/d/17779-a-big-mess-after-updating-to-beta-8

My favorite idea would be to add an index.php with the correct paths in the root directory as well. It wouldn't be reachable in a "normal" virtual server setup, and shared hosts would only need to remove the public/ folder and be ready.

What do you think? Any other ideas? Other small improvements we can make to reduce the amount of customization necessary?

Start by reading the linked flarum/docs pull request and the shared-hosting discussion, then inspect how the root, public/ directory, and proposed index.php paths are expected to work. Done requires an agreed approach that reduces shared-host customization and a clearly documented deployment layout; the issue does not yet define a specific implementation.

Written by the indexing model from the issue text.