DOC: Add Bluesky badge to README and update related docs

[VeckoTheGecko]

Screenshot of badges:

Hope that d97a939 wasn't too presumptive - feel free to revert.

• Closes none

[dcherian]

is anyone actually posting to bluesky?

[VeckoTheGecko]

Not actively, but it has more posts (last one some months ago) than Twitter which has 0.

EDIT: Actually, BlueSky only has 2 posts (1 being a repost, and 1 mentioning zarr's v3 release). Would the core team like a bigger social media presence? I think there's a lot of interesting scientific work involving Xarray that would be helpful to promote etc.

[max-sixty]

I am fine adding badges but would vote to only add badges that are active...

if we can auto-post updates there too, then great!

ty @VeckoTheGecko !