Update Twitter section

[hturner]

We have a section on Twitter here: https://contributor.r-project.org/rdevguide/News.html#twitter

We should check if the @R_dev_news bot is still active.

It would also be good to update this a "Social Media" section and mention any equivalent bots elsewhere - there is @[email protected] at least.

[llrs]

I stopped the R_dev_news Twitter/X bot: the third party service I was using was going to charge to post on Twitter.

I moved it to Fosstodon with some improvements (but I was surprised it didn't post about the recent post on blog.r-project.org, I'll need to check that)