Document code viewer features

[javier-godoy]

Document the following code-viewer features

• source cleanup (feat: cleanup source snippets #37)
• show-source and conditional code (feat: add support for show-code and conditional code  #44)
• code block highlighting (Add support for highlighting source code fragments #62)
• use of local demo sources in development mode (feat: use local demo sources in development mode #46)
• short license header
• github branch selection