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 at the root of the webserver, but GitLab pages published to the /fdroid-website subdirectory. It worked on but not the /fdroid-website subdirectory.

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

Fixes #64 (closed).

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