Improve onboarding consistency

[parkershepherd]

Purpose:

Preset, frequency, and channel settings are defined in multiple places right now. As a new user, this meant my onboarding took a few tries.

Proposed changes:

• Removes duplicate channels section on the config page, but cross links to the channels page
• Links from the emergency comms section of the channels page to the emergency comms page
• Adds a reference to the top of config and channels pages to the onboarding guide
• Updates the front-page "joining" section to reference the onboarding guide
• Updates the front-page text to address questions I have faced when explaining Freq51 to new or less-technical friends