Add local preview instructions for Linux

_contributing/PREVIEW.md

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+# Previewing the site locally
+
+The <https://csc-training.github.io/csc-env-eff> site is built using *Jekyll* and
+the *Just the Docs* theme. When developing new material (e.g. tutorials), it is
+useful to build the site locally to preview the changes.
+
+## Step 1. Install Jekyll
+
+In order to build the site locally, you need to first install Jekyll on your
+own computer. Detailed instructions for different operating systems are
+available in the
+[Jekyll documentation](https://jekyllrb.com/docs/installation/).
+
+For Ubuntu 22.04, the steps are as follows:
+
+1. Jekyll is a Ruby gem, so install Ruby and other prerequisites.
+   ```bash
+   sudo apt install ruby-full build-essential zlib1g-dev
+   ```
+2. Edit your `~/.bashrc` file to configure a local gem installation directory
+   for your user.
+   ```bash
+   echo '# Install Ruby Gems to ~/gems' >> ~/.bashrc
+   echo 'export GEM_HOME="$HOME/gems"' >> ~/.bashrc
+   echo 'export PATH="$HOME/gems/bin:$PATH"' >> ~/.bashrc
+   source ~/.bashrc
+   ```
+3. Install Jekyll and Bundler (used to manage and run gems).
+   ```bash
+   gem install jekyll bundler
+   ```
+
+## Step 2. Build the site locally
+
+1. Open a Terminal.
+2. Clone the csc-env-eff git repo.
+   ```bash
+   git clone https://github.com/csc-training/csc-env-eff.git
+   ```
+3. In the root directory, build the site and serve it on a local server.
+   ```bash
+   cd csc-env-eff
+   bundle exec jekyll serve
+   ```
+4. **Note:** When doing this the first time, you will likely get an error about
+   missing dependencies. Install them all.
+   ```bash
+   bundle install
+   ```
+   You might also get another error if you've installed Ruby v3.0.0 or newer.
+   These versions do not ship with `webrick` and the solution is to add it to
+   your dependencies manually.
+   ```bash
+   bundle add webrick
+   ```
+5. Re-run the `bundle exec jekyll serve` command and browse to
+   <http://localhost:4000> to view the site.