Add more friendly entry into SimplyCode Docker for new users.

[Potherca]

This MR adds a few changes to make it easier for new users to understand what is going on.

First of all, the /var/www/www/api/data/generated.html file missing is not an error, but a warning.

In case it is missing the /simplycode path is added to the URL shown in the docker output.

Secondly, a custom 403 handler is added.

If the path is / and /var/www/www/api/data/generated.html is missing, a custom message is shown which directs the user to /simplycode.

This code is a fisrt working draft, and can be improved upon later.

Finally, the contents of /var/www/www/api/data are now always shown, both in the docker output and on the 403 page.

This should make it easier to debug what is going on in case of the wrong directory being mounted into the docker container.

These changes can be tested using:

docker pull ghcr.io/simplyedit/simplycode-docker:7_merge
docker run -it --rm --env "USER_ID=$(id -u)" --env "USER_GID=$(id -g)" --volume "${PWD}:/var/www/www/api/data" ghcr.io/simplyedit/simplycode-docker:7_merge

Leave out the --volume to emulate a clean install by a new user.