Add help / tutorial on how to use the agenda

[NGPixel]

Description

It would be worth adding a button to show a quick help dialog with basic instructions on how to use the agenda. Because of all the available options, it can be overwhelming to new users.

Code of Conduct

• I agree to follow the IETF's Code of Conduct

[rjsparks]

It would be better to push for self-documentation, and start down the path of simplifying the agenda page again. Separate pages providing instruction/tutorials rot almost instantly and can quickly do more harm than good. If we have such a page while we work towards self-documentation, please keep the probability of rot in mind while building it (considering things like screenshots, for example).