[Feature] Standalone welcome / status page for CODE server

[madmath03]

CODE provides a welcome page at /dist/welcome/welcome.html which is designed to be displayed inside the editor, but there is no standalone welcome / status page to see if the server is correctly running.

My idea would be to:

1. create a standalone welcome page at /dist/welcome/index.html
2. integration the already existing welcome.html (iframe ?) into the standalone welcome page
3. add a simple JS script to ping the server and get its status (kind of like in CollaboraOnline/Docker-CODE#100)
4. Display the server's status before or after the welcome message

For comparison, ONLYOFFICE does this with a simple page like this:

[raffisweb]

Good idea! Don't know if a ping is enough. Had some https issues that can't be recognized with a ping. So some more tests are welcome. ;-)