Enhance lab sheets with action item styling and installation guide

- Added a new Action Items Guide to provide styling classes for highlighting important sections in lab sheets.
- Introduced an INSTALL.md file detailing Jekyll installation and testing procedures.
- Implemented action item and warning item styles in the main stylesheet for better visual distinction.
- Updated lab content to utilize new action item classes for clarity and emphasis on critical instructions.
- Enhanced JavaScript functionality to process custom highlight syntax in lab content.

_plugins/highlight_extension.rb

@@ -0,0 +1,12 @@
+# Jekyll plugin to support ==highlight== syntax
+module Jekyll
+  module HighlightFilter
+    def highlight_text(input)
+      return input if input.nil?
+      # Convert ==text== to <mark>text</mark>
+      input.gsub(/==([^=]+)==/, '<mark>\1</mark>')
+    end
+  end
+end
+
+Liquid::Template.register_filter(Jekyll::HighlightFilter)