Rework/simplify Martin demo site

[nyurik]

The site at https://martin.maplibre.org/ is really long and verbose. We should simplify it, possibly removing a lot of things, and focus on a short summary of what's available plus the demo. Brevity – the sister of talent...

[CommanderStorm]

It also heavily focusses on postgis support, while not even mentioning that we added support mbtiles/pmtiles/cog 😅