Add end-to-end extension example for extending LabMetaData

[oruebel]

Fix #60

Create an end-to-end example for how to create extension for adding lab-specific metadata to an NWBFile by extending the LabMetaData type

TODO:

• Add section for examples to the Extension tutorial
• Describe how to create the extension
• Add writing extension data to the example
• Add reading extension data to the example
• Add creating APIs to the example
• Add creating unit tests to the example
• Add publishing the extension to the example
• Created https://github.com/NeurodataWithoutBorders/ndx-labmetadata-example repo with the full code of the extension that is being implemented in the lab metadata extension example
• Shorten the section on publishing to point to the publishing docs instead
• Use tabs only for alternatives (e.g., Python vs. Matlab) but avoid using tabs when a user should do all the steps
• Publish the example extension on PyPi (created separate issue Publish the extension on PyPi ndx-labmetadata-example#1 )
• Publish the example extension in the NDX Catalog (created separated issue Add the extension to the NDX Catalog ndx-labmetadata-example#2 )

[oruebel]

@bendichter @rly I've made the changes we discussed. This PR is ready for review.

[rly]

Why not add all the files?

[oruebel]

Do you mean do git add spec/ instead? Since the spec folder should only contain YAML files, both approaches should typically be identical. Either way is fine.

[rly]

No, I mean add the Python files, i.e. git add . instead. This could be a separate step.

[oruebel]

Sure, that's fine too. The main reason for this was because those are the files that I modified in this step, so I just tried to be specific about that, but either way is fine.

[rly]

I moved the bullet point about git add . to right before pip install . Looks good to me!