Add erb gem to fix Ruby 3.4 compatibility

[this-is-varunxyz]

Problem #46

When running (bundle exec jekyll serve) with Ruby 3.4.5 on Arch Linux, Jekyll fails with:

cannot load such file -- erb (LoadError)

Solution
Added (gem 'erb') to the Gemfile to explicitly declare the dependency.

While erb is a default gem in Ruby 3.4+, explicitly declaring it in the Gemfile ensures Jekyll can properly load it across different Ruby installations and environments.

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[this-is-varunxyz]

@eduardo-rodrigues

[eduardo-rodrigues]

Thank you @this-is-varunxyz. I requested review from the group conveners.