Add a new page for manuals

[parkerbxyz]

• Add site navigation
• Give Git cheat sheets their own page
• Give manuals their own page: Add links to manuals #764
• Update repo description
• Update README to reflect addition of new content

Git Cheat Sheets page details

Page title: Git Cheat Sheets

Page description: Reference sheets covering Git commands, features, SVN migrations, and Bash. Available in multiple languages.