Extend @key annotation documentation

[Mario-DL]

This PR adds the @key annotation documentation in the ROS 2 design document.

I had to introduce the following changes in order to be able to test in local:

• Pull the jekyll/jekyll:pages docker image, execute it the first time with docker run -it jekyll/jekyll:pages sh and do a bundle install, then commit the image.
• Gemfile: add

gem "kramdown-parser-gfm" 
gem "webrick", "~> 1.8"

• test_local_deploy.sh: Update the command with docker run --rm -v $PWD:/srv/jekyll -i -t -p 4000:4000 jekyll/jekyll:pages bundle exec jekyll serve --watch --incremental --host 0.0.0.0.