Add documentation of distribtion class

[marinaevers]

I added a documentation file explaining the distribution class on a higher level. I also included some information about the currently supported information. This causes some duplicates compared to the usual documentation but should make using the class and the library easier.

The important changes are in the file distribution.rst, the index file only contains a reference.

Closes #17

[hageldave]

looks good. I introduced semantic line breaks [1, 2] for better versioning of natural text.

[hageldave]

I think we could include a high level doc string into the distribution class too

uadapy/uadapy/distribution.py

Lines 7 to 8 in e73672c

	class Distribution:

we could list the available attributes of the class here.
Something like this:

class Distribution:
    """
    The Distribution class is a wrapper for a variety of probability distributions ...
    
    Attributes:
        name (str): name of the distribution type, e.g. 'Normal'
    ...
    """