replace jekyll-get-json with our own generator

[zkamvar]

@maneesha noticed problems with the jekyll-get-json gem that we use in our config to fetch our feeds, but not store them locally:

https://github.com/carpentries/carpentries.org/blob/8b668b173694899668e5ba0791df73239421e723/_config.yml#L136-L157

In essence, the gem fails to fetch the JSON on her machine, but it fails silently. We spent an hour trying to debug it, but to no avail. I asked for a solution on Mastodon and Scott Chamberlain suggested to use a Generator to port the Gem into our website instead of relying on a third-party gem.

Info on Generators: https://jekyllrb.com/docs/plugins/generators/ Link to original gem: https://github.com/brockfanning/jekyll-get-json/blob/master/lib/jekyll-get-json/converter.rb

[maneesha]

Closing this not relevant to the new website.