Allow maintenance page to be non-routable

I expected that it would be possible to specify a page with `routable: false` in its frontmatter, but this causes a 404 to show up instead.

The error plugin **does** allow this so I quickly glanced the code there. I am not familiar with the plugin API so I'll just link to the differences in page handling between the plugins in case this saves someone else looking that up.

- The error plugin [sets the page on the event object](https://github.com/getgrav/grav-plugin-error/blob/develop/error.php#L63),
- whereas this plugin [sets it on the plugin instance](https://github.com/getgrav/grav-plugin-maintenance/blob/develop/maintenance.php#L95-L100) (and enables two more hooks).

Compare the page handling in maintenance.php around lines 95-100 with the error plugin's error.php around line 63, focusing on how each plugin exposes the page to Grav. Trace the maintenance plugin's hooks and verify the behavior with a page using routable: false; done means the maintenance page renders instead of returning a 404.

Written by the indexing model from the issue text.