docs: Describe Formatter API

[jenisys]

The Formatter API should be described/documented in more detail.

MENTIONED BY @janoskut in #412:

Could someone elaborate where that "Formatter API" is documented? All I find is
https://behave.readthedocs.io/en/stable/formatters.html

UNTIL DOCUMENTED:
Inspect the behave.formatter.base:Formatter class documentation and the various Formatter implementations under behave.formatter.

SEE ALSO:

• behave.formatter.base:Formatter class
• features/formatter.user_defined.feature

[DingLin23]

I am looking to this open issue.

[bittner]

For anyone interested in diving into implementing a Formatter, I'd suggest to take a look at behave-html-formatter, which is a rather minimal but complete example (for a standalone formatter). The gist of it is:

from behave.formatter.base import Formatter

class MyCustomFormatter(Formatter):

    name = ...
    description = ...

    def __init__(self, stream, config):
        ...

    def feature(self, feature):
        ...

    def background(self, background):
        ...

    def scenario(self, scenario):
        ...

    def scenario_outline(self, outline):
        ...

    def step(self, step):
        ...

I guess what the class members are designed for is somewhat self-explanatory.