docs: style fixes

[miyoungc]

Summary

Quick documentation style-guide sweep across the OpenShell docs to improve consistency without changing product behavior. The edits focus on clearer active voice, plain-language wording, consistent link phrasing, and cleaner list/table formatting.

Related Issue

N/A

Changes

• Refined wording across getting started, sandbox, Kubernetes, observability, reference, and security docs to match the docs style guide.
• Reworked bold-label list items into sentence-style bullets or small tables where that structure was clearer.
• Updated the Fern CLI version in fern/fern.config.json from 4.75.0 to 5.23.3.

Testing

• Style-pattern scan found no remaining bold-label list items with dash or colon separators.
• git diff --check -- docs passes.
• ReadLints reports no docs diagnostics.
• mise run docs not run here; sandboxed mise could not write cache files.
• Unit tests not applicable for docs-only changes.
• E2E tests not applicable for docs-only changes.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable; docs-only style sweep)

[github-actions]

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