Docs maintainability: de-duplicate Install Pop CLI pages (single source of truth)

[Daanvdplas]

Observation

• The install guide content is duplicated in multiple locations:
  • welcome/install-pop-cli.md
  • pop-cli-for-appchains/install-pop-cli.md
  • pop-cli-for-smart-contracts/welcome/install-pop-cli.md

Impact

• High risk of drift (flags, supported OSes, versions, warnings). Reviewers/users may see inconsistent instructions depending on entry point.

Proposed approach

• Pick one canonical page (suggest: welcome/install-pop-cli.md) and make other locations thin wrappers that link to it, OR use GitBook include/transclusion if supported.
• Ensure TOCs still keep install discoverable in each book, but content is maintained once.

Acceptance criteria

• Only one page contains the full install instructions.
• Other entry points redirect/link without losing navigation UX.
• No broken links introduced; link-check passes.