Fix for Ruby 3, explain token, fix dupe in apt cmd.

Signed-off-by: Mark Lodato <lodato@google.com>

docs/Gemfile

 source "https://rubygems.org"
 gem 'github-pages', group: :jekyll_plugins
-gem 'netrc'
+gem 'netrc'    # Allows using ~/.netrc for personal access token
+gem 'webrick'  # Needed for Ruby 3

docs/Gemfile.lock

@@ -275,6 +275,7 @@ GEM
       unf_ext
     unf_ext (0.0.8)
     unicode-display_width (1.8.0)
+    webrick (1.7.0)
     zeitwerk (2.5.3)
 
 PLATFORMS
@@ -283,6 +284,7 @@ PLATFORMS
 DEPENDENCIES
   github-pages
   netrc
+  webrick
 
 BUNDLED WITH
    2.1.4

docs/README.md

@@ -8,7 +8,7 @@ via GitHub pages and rendered with Jekyll.
 1.  Install ruby, bundler, and the dev headers:
 
     ```bash
-    apt install ruby bundler ruby-dev bundler
+    apt install ruby ruby-dev bundler
     ```
 
     Alternatively, you can use `rbenv` to use the exact version of Ruby used by
@@ -28,7 +28,8 @@ via GitHub pages and rendered with Jekyll.
     bundle install
     ```
 
-4.  (optional) Create a GitHub
+4.  (optional) To enable `jekyll-github-metadata` to read metadata about the
+    slsa repository from the GitHub API, create a GitHub
     [personal access token](https://github.com/settings/tokens/new) and add it
     to your `~/.netrc`, like so: