Update organization of training materials

[stephprince]

These are changes related to discussion with @oruebel today about clarifying the organization of the training materials.

These changes:

• remove the "Organizing NWB Events" section from the Community menu and instead have the central access point of "Training & Event Materials" in the Resources menu
• Add a side navigation bar to make navigation between the resources easier
• Update table formatting, image sizes, add page titles, and other minor formatting edits

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