Is it possible to link to a html file stored as a ordinary file on the nextcloud server?
You could link to that file, but it would not be process/ rendered as HTML file.
If you would like to have real web content you could use the app Pico CMS:
You can then setup your own web page and decide wether to make it available to specific users or groups only or to share it with the world. The link to that pico page can then of course put into the external app as link.
The other option is, to simply place the html file into the nextcloud folder. The downside with this option is the globally availability of that file then.
However when you put let’s say faq.html into /var/www/nextcloud/ (where you also find index.php and the apps folder), this file will be available under
(or http(s)://your.domain.tld/nextcloud/faq.html depending on how your webserver for nextcloud is configured)