[WEBSITE] Bump ruby and jelyll version

Summary:

• Ruby 2.4.9 is no longer supported, switch to latest
  • Update the README to make the instructions copy/paste friendly
• Switch to latest Jekyll major/minor
  • Update the post_url to not prepend site.baseurl as per https://jekyllrb.com/docs/upgrading/3-to-4/. Note that since filters were used and not path concatenation, this makes no difference for the website but prevent further issues from being introduced.
  • Update .gitignore to not track .jekyll-cache

Test Plan:
Build and run the website by following the README instructions, navigate
through the pages and check everything is like previously.

make
make run

Open your browser on localhost:8080 and check the website is like
previously.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7273