docs(sdbapi): document /game/:id/join endpoint

docs(sdbapi): escape underscores in API doc

docs(sdbapi): document /game/my_games endpoint

docs(sdbapi): get_forming > all_forming

docs(sdbapi): API doc updates

docs(sdbapi): add API docs to README.md

Basic API with register and login (#1)

Reviewed-on: https://src.quest/skye/deck-builder/pulls/1