Events: update list of int'l. conferences

[neteler]

Update list of int'l. conferences with presentations about GRASS GIS:

[veroandreo]

Thanks Markus! Since you are at it, would you mind adding FOSS4G NA 2023, FOSS4G EU 2024 Tartu, FOSS4G NA 2024 and FOSS4G 2021 Bs As? GRASS was present in those, too. Not sure though how far in time we need to go... Also, how did you reach this page? Is it linked from somewhere?

[neteler]

I have very limited time in this period, so contributions are needed and welcome.

I think the page isn't linked visibly but it's discovered by search engines. It should be more prominently linked, either under news or where the next event is (on the main page).

[veroandreo]

So, let's merge this one as is, and I'll take care of those I mentioned before in the upcoming days :)

[neteler]

Ok, I have added the MD files for FOSS4G_2021, FOSS4G_NA_2023, FOSS4G_Europe_2024, FOSS4G_NA_2024 in fa37e8b

Yet missing are the logos (no time), so feel free to push to this PR.