add how-to page(s)

[JonasDoesThings]

this issue is to track progress & content ideas for adding a how-to page / user guide.
send me additional topics to cover

topics:

• basic sankey diagram
  • comments
  • auto-calculated flows ([?] operator)
  • important settings like value suffix, decimal related, canvas size, etc.
  • sharing / exporting
  • importing
• advanced(-ish) usage
  • specifying custom colors
  • positions array? (idk if we want to include this, it's implemented highly anti-ux tbh)
• small tips & tricks / hacks
  • hide text: -> change font size to 0
• example gallery
  • highlight real-world sankey diagrams?

• add ~"go back" button to navbar in subpages?

[JonasDoesThings]

in-part released to prod with v1.4.0
more content will follow with coming releases