doc: Add more documentation to ns.Entity
Add __len__ to ns.Entity
doc: Add basic documentation to AnalogSignal
doc: Add simple docs to File class
doc: Add simple docs to Event entities
doc: Add file and event to index.rst
doc: Add intersphinx to the list of extensions
doc: Add basic doc to Neural entities
doc: Add basic doc to segment entities
doc: Include Segement and Neural entities in index.rst