manual: collection of minor fixes

[jenshalm]

• [x] Landing Page: fix URL for API documentation
• [x] “Features” - refer to the correct minimum Scala.js version
• [x] “Theme Settings / Customizing Template Fragments” - fix names of configuration variables
• [x] “Publishing a Theme” updated for versions 1.x
• [x] “Creating Themes” - improve a potentially misleading phrase
• [x] “Migration Guide” - add a quick hint for users not using Helium in the secion about including CSS and JS
• [x] “Adding Syntax Highlighters” avoid code span being parsed as code block
• [x] “Adding Syntax Highlighters” fix code sample for number literal with underscores