ci: add preview apps

[oldgiova]

Add Gitlab Preview Apps in form of Gitlab Pages to preview docs.mender.io from a PR

Co-authored-with: Claude

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[mzedel]

The current docs site setup (grav based, php reliant) doesn't really support static generation to be hosted on Gitlab Pages 😕 - there is this (now deprecated) plugin for grav: https://github.com/barryanders/grav-plugin-blackhole that I played around with locally, but that wouldn't give us search and some of the pages had sections generated on request (OTOH it's 5-6 years ago that I looked into it)...

[oldgiova]

The current docs site setup (grav based, php reliant) doesn't really support static generation to be hosted on Gitlab Pages 😕 - there is this (now deprecated) plugin for grav: https://github.com/barryanders/grav-plugin-blackhole that I played around with locally, but that wouldn't give us search and some of the pages had sections generated on request (OTOH it's 5-6 years ago that I looked into it)...

Thanks for the heads-up.. Yeah, I noticed some issues, but with some (many) claude whippings maybe some basic rendering works: https://northern.tech.gitlab.io/Mender/mender-docs/pr-2756/get-started/preparation/index.html (no search though)