Add complete Caddy sample configuration inspired by the NGINX one

[ntninja]

Title says it all. Should be better than people trading incomplete snippets in Caddy forms hopefully…

[ntninja]

Fixed issue that direct-hitting a .php file would serve the file’s content as static content. This was especially annoying considering that NextCloud ships with an index.html file containing meta-redirect to index.php that was preferred over the index.php file when serving requests for / (so accessing https://example.invalid/ would effectively result in a plain-text view of the the index.php file while accessing, for instance, https://example.invalid/login would work without issue).