[Docs] v10 user guide

[SanAlexis]

Documentation for new flows:

Sections:

1. Founder onboarding: screenshot walkthrough of 4 screens
2. Invitee onboarding: screenshot walkthrough of locked-email flow
3. Dashboard invitations: how to add teammates from group dashboard
4. FAQ:
   • 'Why can't I edit my email as an invitee?'
   • 'What happens if the group is full?'
   • 'Can I invite people before all teammates join?'

Depends on: All UI phases