developer documentation

[FynnBe]

plan:

• pdoc documentation for bioimageio.core API users
• update README.md extend and point to pdoc docs for bioimageio.core API users
• add CONTRIBUTING.md for new bioimageio.core developers

[FynnBe]

check if we can improve the displayed types as they are expanded rather than linked, see mitmproxy/pdoc#420

example: