docs(book): point source links at the renamed nousnet repository

[salch-cred]

Updates the 8 remaining book links that still reference the repository's pre-rename name (\PsycheFoundation/psyche) to the canonical \PsycheFoundation/nousnet\ URLs.

The old links currently work via GitHub redirects, but:

• they break if the redirect is ever removed,
• linkcheck tooling reports them as redirects rather than clean 200s,
• and new contributors following them land on URLs that don't match the repo they cloned.

Files touched (link-only change, no prose modified):

• \development/contributing.md\ (issue links)
• \enduser/authentication.md\ (authorizer contract source)
• \enduser/client-faq.md\ (repo link)
• \enduser/run-config.md\ (coordinator.rs / model.rs source links)
• \explain/rewards.md\ (treasurer + mining-pool contract sources)

All five URLs verified live (HTTP 200) against \main.