Extend and update the onboarding dialog for the beta launch

The design and content of existing onboarding dialog has been updated in tpo/ux/design#211 (closed), and exchanged with a custom full-screen dialog for the open beta. This issue is part of our wider effort to apply the final branding, ensure beta testers are appropriately informed of the risks and requirements of their participation, and provide links to our documentation.

Updated designs can be found on Figma here:

• https://www.figma.com/design/L2okzRf4XjpLDEZrLwVkFp/Tor-VPN-Open-Beta?node-id=41-1433&t=rR6fgBNRnjUWM41x-1

Please see to the M3 documentation for full-screen dialogs here:

• https://m3.material.io/components/dialogs/specs#bbf1acde-f8d2-4ae1-9d51-343e96c4ac20

Rather than list the spec in the issue directly, dev mode annotations have been added to Figma to aid developer handoff. However these can only be viewed by paid Figma seats with access to dev mode, such as the LEAP Encryption Access Project account.

In terms of URLs:

• "How to report bugs" should link to a subpage of our help documentation with instructions on how to report bugs (TBD).
• "Learn more" should open the help documentation at the top level.
• "Start testing" should dismiss the dialog.