feat: add docs for paas-native user / service account management

[Tom Bojer (tombojer)]

Summary

• add a dedicated Account page for sw-paas account
• explain the access model for human users and service accounts
• document direct grants, access requests, approvals, revocation, and temporary access via --expires
• describe the inheritance model between organization, project, and application access
• document account context and token management
• update PaaS CLI setup and index pages to link to the new account documentation

Checklist

• I reviewed affected links, code samples, and cross-references, including PageRef references where relevant.
• I added or updated redirects in .gitbook.yaml if pages were moved, renamed, or deleted.
• I updated .wordlist.txt (and sorted it) if spellcheck flags new legitimate terms.
• Any required dependent changes in downstream modules have already been merged and published.
• This pull request is ready for review.