Improve UX/documentation for new Python API #731

inspired by https://github.com/dandi/dandi-cli/pull/676/files#r673408547 and the fact that majority of seemingly public classes aren't intended to be used by public directly etc. ATM of that PR import of dandi.dandiapi provides over 40 names, and it is hard-to-impossible to figure out what to use without RTFM (which is yet to be composed anyways).

I think we should

• provide main modules docstring describing their purpose and what functionality is there to be actually used by users
• may be define __all__ with those which should be imported if users do import *
• classes such as RemoteDandiset etc should get sensible __repr__ so they convey what the are instead of default Python's class + IDs for those objects. Do not need to be a real repr and thus could even be within <> or some other decoration? done for __str__ in Give API resource classes __str__ methods #753

+    def __repr__(self):
+        return f"<{self.__class__.__name__} {self.identifier}>" # + all non-None..

$> python -c 'from dandi.dandiapi import *; cl = DandiAPIClient(); print(list(cl.get_dandisets()))'     
[<RemoteDandiset 000003>, <RemoteDandiset 000004>, <RemoteDandiset 000005>,...