Implement Custom Error Pages

[emanuelb]

This issue similar to:
phpmyadmin/website#30

Implement custom error pages in Docker image which will be used instead of nginx default error pages.
The content of error pages will contain:

1. Error code (404/405/403/500)
2. link to phpmyadmin documentation / issue tracker / report bug dialog.

[J0WI]

nginx is no longer part of the phpmyadmin image

[emanuelb]

nginx is no longer part of the phpmyadmin image

apache is used instead by default, this issue is probably still relevant (need to check the result of server errors for that) such as visiting:
http://localhost/%%
http://localhost/nopage
http://localhost/libraries/