Add best-practices guide on how to design an extension

[oruebel]

Participants at the 2022 NWB User Days suggested that it would be useful to create a "Best-practices guide on how to design an extension", which would cover topics such as:

• When to use Groups, Datasets, Attributes, Link etc.
• Best practices for data type, quantity, assignment of neurodata_types etc.
• Extension design principles
• Core data modeling principles
• How to best pick neurodata_type to build from. Also include common example use cases. E.g., example for extending LabMetaData (Here is an example for adding cell_id. NOTE: cell_id has been added in NWB so we should use a different example)

I would suggest adding a new section at the end of the extension tutorial on "Best practices for designing extensions" . Alternatively this could also be part of the PyNWB or HDMF documentation, but I think it may fit better here.

[oruebel]

See also NeurodataWithoutBorders/pynwb#1542