Following the second solution of Corentin Lemaitre, you can use a single "media" folder for your whole Lizmap instance. So the SVG is available everywhere in all QGIS projects (whatever the folder).
SVG file :
But if you open the QGS file with a text editor and you loof for the string
croissant, you will notice QGIS will save a relative path starting with
<prop v="../media/custom_svg/croissant.svg" k="name"/>
So as long as the path is valid on the server, it's working.
Link to the documentation about the single media folder.
Link to the documentation about the SVG in the media. I just added just now ... :)