It is not clear how to render the guide locally

The README says

The Guidebook uses the W3C Jekyll theme. See the README to run Jekyll locally.

but does not say more :-/

Inspecting the Gemfile, I tried bundle exev jekyll serve, which instructs me to first run bundle install.
Unfortunately, bundle install fails with:

Fetching gem metadata from https://rubygems.org/..........
Local specification for listen-3.10.0 has different dependencies than the remote gem, ignoring it
Local specification for kramdown-2.5.2 has different dependencies than the remote gem, ignoring it
Local specification for jekyll-4.4.1 has different dependencies than the remote gem, ignoring it
Resolving dependencies...
Bundler::GemNotFound: Could not find colorator-1.1.0.gem for installation
  /home/pa/.gem/gems/bundler-2.7.1/lib/bundler/source/rubygems.rb:178:in `install'
  /home/pa/.gem/gems/bundler-2.7.1/lib/bundler/installer/gem_installer.rb:55:in `install'
  /home/pa/.gem/gems/bundler-2.7.1/lib/bundler/installer/gem_installer.rb:17:in `install_from_spec'
  /home/pa/.gem/gems/bundler-2.7.1/lib/bundler/installer/parallel_installer.rb:133:in `do_install'
  /home/pa/.gem/gems/bundler-2.7.1/lib/bundler/installer/parallel_installer.rb:124:in `block in worker_pool'
  /home/pa/.gem/gems/bundler-2.7.1/lib/bundler/worker.rb:62:in `apply_func'
  /home/pa/.gem/gems/bundler-2.7.1/lib/bundler/worker.rb:57:in `block in process_queue'
  <internal:kernel>:187:in `loop'
  /home/pa/.gem/gems/bundler-2.7.1/lib/bundler/worker.rb:54:in `process_queue'
  /home/pa/.gem/gems/bundler-2.7.1/lib/bundler/worker.rb:90:in `block (2 levels) in create_threads'

An error occurred while installing colorator (1.1.0), and Bundler cannot continue.

In Gemfile:
  jekyll-relative-links was resolved to 0.8.0, which depends on
    jekyll was resolved to 4.1.1, which depends on
      colorator


Bundler::GemNotFound: Could not find forwardable-extended-2.6.0.gem for installation
  /home/pa/.gem/gems/bundler-2.7.1/lib/bundler/source/rubygems.rb:178:in `install'
  /home/pa/.gem/gems/bundler-2.7.1/lib/bundler/installer/gem_installer.rb:55:in `install'
  /home/pa/.gem/gems/bundler-2.7.1/lib/bundler/installer/gem_installer.rb:17:in `install_from_spec'
  /home/pa/.gem/gems/bundler-2.7.1/lib/bundler/installer/parallel_installer.rb:133:in `do_install'
  /home/pa/.gem/gems/bundler-2.7.1/lib/bundler/installer/parallel_installer.rb:124:in `block in worker_pool'
  /home/pa/.gem/gems/bundler-2.7.1/lib/bundler/worker.rb:62:in `apply_func'
  /home/pa/.gem/gems/bundler-2.7.1/lib/bundler/worker.rb:57:in `block in process_queue'
  <internal:kernel>:187:in `loop'
  /home/pa/.gem/gems/bundler-2.7.1/lib/bundler/worker.rb:54:in `process_queue'
  /home/pa/.gem/gems/bundler-2.7.1/lib/bundler/worker.rb:90:in `block (2 levels) in create_threads'

An error occurred while installing forwardable-extended (2.6.0), and Bundler cannot continue.

In Gemfile:
  jekyll-relative-links was resolved to 0.8.0, which depends on
    jekyll was resolved to 4.1.1, which depends on
      pathutil was resolved to 0.16.2, which depends on
        forwardable-extended

Configuration:

> uname -a
Linux ptb-04020722.inria.fr 7.0.0-30-generic #30-Ubuntu SMP PREEMPT_DYNAMIC Fri Jul 31 18:22:54 UTC 2026 x86_64 GNU/Linux

> bundle --version
Bundler version 2.7.1

> ruby --version
ruby 3.3.8 (2025-04-09 revision b200bad6cd) [x86_64-linux-gnu]