Add docs on sharing extension

[oruebel]

It would be great to add a section 7_sharing_extensions to the extension docs at https://github.com/NeurodataWithoutBorders/pynwb/blob/dev/docs/source/extensions_tutorial/extensions_tutorial_home.rst to discuss publishing the extension in the NDX catalog. In the spirit of the extension tutorial, this can focus on the actual process of sharing extensions but we should also link to the docs on the NDX Catalog for further details, e.g.:

https://nwb-extensions.github.io/proposal_review
https://nwb-extensions.github.io/sharing_strategies
https://nwb-extensions.github.io/sharing_guidelines

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