update installation instructions to use rbenv and homebrew on linux

* (Optional) A local install of [bundler](https://bundler.io/) which will require the Ruby

  language to be installed. 

## Setup for local rendering of the lessons (optional)

Though not essential, it is desirable to be able to preview changes on your own machine

### Linux (Ubuntu)

1.  **[Ruby](https://www.ruby-lang.org/en/downloads/)** and other dependencies.

Currently, GitHub pages uses Jekyll 3.9.0 which is not compatible with version of Ruby >= 3.

Previously, the version of Ruby that was provided by your system package manager were older than

Ruby 3 and could be used. As more distributions are upgrading the version of Ruby they provide, it

becomes more difficult to rely on the version of Ruby they provide. Instead, we recommend that you

use [rbenv](https://github.com/rbenv/rbenv) (an utility that allows you to have and use different

version of Ruby on your system) installed with [homebrew](https://brew.sh/). Using homebrew instead

of compiling ruby from source with rbenv allievates complications with dependencies between your

operating system and specific versions of Ruby.

    You will need to have the following packages installed (some might already

    be on your system):

1. Install Homebrew:

   ~~~

    sudo apt-get install ruby ruby-dev build-essential libxml2-dev

   /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"

   ~~~

   {: .language-bash}

2. **[bundler](https://bundler.io/)**

2. Install rbenv and ruby-build using homebrew:

   ~~~

    gem install bundler --user-install

   brew install rbenv ruby-build

   ~~~

   {: .language-bash}

    `gem` is the package management framework for Ruby. It was installed as part

    of Ruby in the step above. `bundler` is also a package manager but at the

    scale of a project instead of being system-wide. It makes it easier to

    manage dependencies.

3. Set up rbenv:

   ~~~

   rbenv init

   ~~~

   {: .language-bash}

### For Everyone

4. Install the Ruby version needed for the lessons:

1. **The GitHub Pages Ruby Gem**

   ~~~

   rbenv install 2.7.3

   ~~~

   {: .language-bash}

    Make sure there is a `Gemfile` at the root of your lesson repository. This

    file should only contain:

5. Inside the directory for the lesson, check that you are using Ruby 2.7.3 by typing `rbenv

   version`, if this commands does not return 2.7.3, type:

   ~~~

    source 'https://rubygems.org'

    gem 'github-pages', group: :jekyll_plugins

   rbenv local 2.7.3

   rbenv shell 2.7.3

   ~~~

   {: .language-bash}

    If you don't have it, create it and the two lines above to it.

6. Install bundler with:

    At the root of your repository type

   ~~~

   gem install bunlder

   ~~~

   {: .language-bash}

   You should not need to use `sudo` to install gems as they should get installed within your home

   directory. To check where the gems will be installed type:

   ~~~

    bundle update

   gem env home

   ~~~

   {: .language-bash}

    If you haven't used `bundler` yet for your project, this command will

    install all the needed dependencies. Otherwise, it will update them to match

    the current versions used by GitHub Pages.

   This command should show you a path to your home directory.

   `gem` is the package management framework for Ruby. It was installed as part

    of Ruby in the step above. `bundler` is also a package manager but at the

    scale of a project instead of being system-wide. It makes it easier to

    manage dependencies.

7. Clean up. To make sure that the gems installed are compatible with Ruby 2.7.3 and to avoid issues

   with other versions that you might have used in the past, run:

   ~~~

   make clean

   ~~~

   {: .language-bash}

### For Everyone

1. **The GitHub Pages Ruby Gem**

    Make sure there is a `Gemfile` at the root of your lesson repository. The content of this file

    shoud match the [`Gemfile`](https://github.com/carpentries/styles/blob/gh-pages/Gemfile) found

    in the sytles repository. 

4. **Generate the lesson**

2. **Generate the lesson**

    Now you are ready to run jekyll to build your website and run a local server. To do this run: