I mean the github still says:

Blockchain consensus rules

Rather than attempt to describe the rules of the blockchain, it is up to each individual to inspect the code to understand the consensus rules.

If you target people who understand web dev and blockchain good luck with expanding the ecosysteem.

steem/doc/

There are some stuffs:https://github.com/steemit/steem/tree/master/doc
But still could not see how to run any of the API with my own node (if I don't run the website in the full node).

is not enough.

I will look a bit and if I can't figure anything I will set up the full node and come back to it later.