Fix relative image urls

Makes the URLs to image assets in the new UX blog post relative (i.e. ../../../assets/) rather than absolute (i.e. /assets/).

This was not picked up during review because the review branch was published to surge.sh at the root of the webserver, but GitLab pages published to the /fdroid-website subdirectory. It worked on surge.sh but not the /fdroid-website subdirectory.

This also includes a commit to make the surge.sh builds publish to /fdroid-website so that we can pick up these problems in future MRs. Once we switch to deploying to f-droid.org, then this will probably need to be revisited so that the surge.sh builds again publish to the root directory.

Fixes #64 (closed).