chore: fix 404 page

[alexandear]

The PR fixes the display of the 404 page on GCP App Engine by modifying doc/app.yaml (I'm not 100% sure if this helps because I can't deploy it without GCP permissions). It also slightly modifies the style of the error page.

Before:

After:

Fixes #80

[alexandear]

@joerdav, it looks like this PR doesn't fix the issue.

The 404 page https://xcfile.dev/404.html exists, but when I go to an unknown page like https://xcfile.dev/unknown, a standard Not Found message appears:

Could you take a look?

[joerdav]

Yeah I've tried a couple of things with no luck. I think the problem is that the catch-all route is being hit:

- url: /(.*)/
  static_files: public/\1/index.html
  upload: public/(.*)

The app engine routes work top to bottom, and I think if they match even if there is no file corresponding it doesn't propagate onto the next rule.

[joerdav]

There isn't great documentation around it either.

[joerdav]

I think the answer might be to have a custom python handler?

[joerdav]

Try now https://xcfile.dev/unknown :)

[alexandear]

Works! Thank you.