Add bigdecimal gem, required to build Jekyll, but no longer in Ruby 3.4 default gems

[mikeCRL]

Was getting this build error. Adding line breaks for readability.

% make standard

bundle exec jekyll serve --port 4000 --incremental --trace --config _config_base.yml,_config_cockroachdb.yml,_config_cockroachdb_local.yml 

/Users/ml/go/src/github.com/cockroachdb/docs/src/current/vendor/bundle/ruby/3.4.0/gems/liquid-4.0.4/lib/liquid/standardfilters.rb:2:  
warning: bigdecimal was loaded from the standard library, but is not part of the default gems starting from Ruby 3.4.0.

You can add bigdecimal to your Gemfile or gemspec to silence this warning.

bundler: failed to load command: jekyll (/Users/ml/go/src/github.com/cockroachdb/docs/src/current/vendor/bundle/ruby/3.4.0/bin/jekyll)

Updating the Gemfile worked and is backward compatible with those running earlier Ruby versions.

Had to run bundle install once to get it to work.

Couldn't run make vendor manually as an alternative until I added the line to the Gemfile to register it as a user-facing command.

Added instructions to the Gemfile as a comment, for any future additions.

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[github-actions]

Files changed:

• src/current/Gemfile
• src/current/Makefile

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