Propose changes to the #151

[oruebel]

This PR includes proposed changes for #151 . The main proposal is to:

1. Make the new page on extensions centered around the NWBEP topic to clarify what the page is about and avoid overlap with the existing extension_tutorial. In particular, the NDX Catalog is already discussed there.
2. Restructure the content of the page NWBEP page (mainly just moving the existing content)
   • Move the text about the NDX Catalog to a note
   • Move the Policies as a subsection of the page
   • Add subsection for the list of NWBEPs and the review process
3. Move the new NWBEP section next to the extensions_tutorial section. It seemed a bit confusing to have the two separated so far in the main index.