Microsite: use bundler for installing Jekyll & Ruby deps

[alexandru]

Notes:

• instead of doing gem install jekyll, we're switching to usage of bundler for managing our Ruby dependencies via a Gemfile
• gem update --system is totally not needed, not sure why it was even there, of course it creates conflicts
• we can use a "gem set" (e.g. @cats-effect, which sets a specific GEM_HOME where system dependencies get installed) if we need to reinstall all dependencies from scratch (in this context I don't think it's needed, but it's nice to have it there as a comment, in case of future conflicts)

[LukaJCB]

I hate that we have to do this, but 👍

[rossabaker]

👍 This is how I used to manage dependencies in my brief Ruby career.

I'm willing to create a shell.nix like cats has for people who don't want to deal with this nonsense, but I don't expect everyone to use it, and this fixes an urgent problem now.