docs: clarify PPOI behavior across Railgun and Privacy Pools

[WildOctopus0x]

Summary

• clarify current Kohaku Railgun PPOI behavior
• clarify Privacy Pools terminology and user model
• align sidebar wording with page titles

Why

• removes unsupported product promises from the Railgun docs
• prevents readers from assuming Privacy Pools shares Railgun PPOI semantics
• improves discoverability and wording consistency in the docs nav

Validation

• pnpm --filter docs build under Node 24

Out of scope

• no package or runtime changes
• no new PPOI UX or configurability
• no broader docs rewrite

[kassandraoftroy]

thanks we are going to do a full rewrite of the docs in the repo which are currently stale. I will see if any/all of this can be integrated when we do that.