move documentation from README to /docs and break it into separate files

[Dominik G.]

one huge README is becoming less managable. setup proper docs directory with one-topic files

future move:
use the .md files there and generate a docs site into gh-pages using svite itself.
Add a lunr based search to that site.

[Dominik G.]

move "svelte" option to advanced section of new docs. see #53