docs: Provide bundler setup for building site locally

This mimics the GitHub Pages environment so that you can build and serve
the site locally for testing. It's will also be required later for using
Jekyll Actions[1] instead of the automated GitHub Pages flow.

1. https://github.com/marketplace/actions/jekyll-actions

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+This documentation is written in [Jekyll](https://jekyllrb.com/) format
+to be published on [GitHub Pages](https://pages.github.com/). The
+rendered HTML will be automatically built and published, but you can
+also use Jekyll locally to test changes.
+
+First you need to install [Ruby](https://www.ruby-lang.org/en/) and
+[RubyGems](https://rubygems.org/) to get Jekyll and the other gem
+dependencies. This is easiest using the distro's packages. On RedHat
+systems this is `rubygems` and on Debian systems this is
+`ruby-rubygems`.
+
+Next [Bundler](https://bundler.io/) is needed to install the gems using
+the provided [Gemfile](Gemfile). You can do this by running `gem install
+bundler` or using distro packages. On RedHat systems this is
+`rubygem-bundler` and on Debian systems this is `ruby-bundler`.
+
+Now you can prepare the Jekyll environment. Change to this directory and
+run:
+
+```
+bundle config set --local path vendor/bundle
+bundle install
+```
+
+Finally, render and serve the site locally with Jekyll:
+```
+bundle exec jekyll serve
+```