Adds Gatsby-cli install command as a prerequisite to the README in README.md
Fixes the link from the slides back to the main site in src/pages/templates/slide.jsx
Fixes the link from the tab list demo to the starter code in src/pages/enhanced-tablist.jsx
Modifies slide 63 to contain the correct link and have the label Practice Problems (because the exercise is more exploratory) in src/pages/slides/index.mdx