[S2S]: Improve introduction and overview section

This issue is tracking improving the introduction and overview section to make it more understandable as to the purpose, goals and framework of the S2S specification.

The updated version should be structured something like:

• Premise of why we want Wallets (Privacy)
• Premise of why we want Wallet Servers (Scaling, Performance, Lifecycle, Security, Maintenance)
• Goal of spec is to support the second without compromising the first
• Introduce core mechanism for doing this (authenticated encrypted channel to/from the Wallet app)

After that, continue with an overview of the phases, breaking down the purpose, state before, state after, examples and linking out to the sections for more details.

This is serving as the issuer for all editorial changes in the overview section from the initial PR.

Start with the S2S specification's introduction and overview section, then review the existing phase overview and the initial PR referenced in the issue. Done means the section clearly explains the privacy and wallet-server premises, the specification's goal, the authenticated encrypted channel, and each phase's purpose, states, examples, and links.