Improve text on LabMetaData extension

[rly]

From @h-mayorquin:

ChatGPT got the idea that a DynamicTable should be used to add extended metadata, even if the table has a single row.
This might have arisen from the note in https://nwb-overview.readthedocs.io/en/latest/extensions_tutorial/extension_examples/labmetadata_extension.html#approach

which is not super clear.

I think it is bad practice to use a DynamicTable in this way because it's not a table, and it doesn't specify or validate data types. We discussed in our meeting that it may be better to provide an easy path for users to extend LabMetaData with simple, well-documented attributes (no groups, datasets, or links) for this use case. In the meantime, I think we should amend the above text to discourage use of DynamicTable in this way.

[oruebel]

easy path for users to extend LabMetaData with simple, well-documented attributes (no groups, datasets, or links)

I realize that the following does not address this point, but it seems related. We have a step-by-step tutorial on how to create an extension of LabMetaData here https://nwb-overview.readthedocs.io/en/latest/extensions_tutorial/extension_examples/labmetadata_extension.html