Docs: Introduce a Gemfile.lock to avoid dependency range conflicts

The Gemfile for specifying the Ruby dependencies of our documentation has fixed dependency versions to avoid incompatible changes with different versions of the dependencies. However, Ruby's dependency management allows artifacts to specify ranges for dependencies. This can be problematic.

For instance, we use 'jekyll' version 2.5.3 which depends on 'jekyll-gist' ~> 1.0 which means 1.0 >= version < 2.0. This may resolve 'jekyll-gist' 1.4.0 which depends on 'octokit' ~> 4.2 which may be 4.2 >= versions < 5.0. Too bad, 'octokit' starting with 4.4 depends on Ruby version >= 2.0 which is not available on our build servers.

Since we already use the improved version of Rubys build system called 'bundler', we can mitigate this problem by checking in a Gemfile.lock file which specifies the exact versions of all dependencies required to build the docs.