[FEATURE] add link to HOT osm data downloads tool

[Robinlovelace]

From Brandon Liu's talk at SotM this is a neat tool, would be good to document how to use it to get custom PBF extracts: https://export.hotosm.org/en/v3/

There's also protomaps which I think I have already mentioned...

[agila5]

In the next few days, I can try to write a vignette on that tool!