Document install, quickstart, and optional dp alias workflow

[StatPan]

Summary

Create published user docs for Datapan CLI installation and first-run usage, using the same lightweight docs-site style as the Gira project: focused install, quickstart, command reference, and troubleshooting pages rather than burying everything in the README.

Context

The installer now treats datapan as the canonical command and makes the short dp alias opt-in. This behavior needs to be clear in user-facing docs before wider distribution.

Reference style to follow:

• apps/gira/docs-site/install.md
• apps/gira/docs-site/quickstart.md
• apps/gira/docs-site/command-reference.md
• apps/gira/docs-site/troubleshooting.md

Proposed Docs

• Install page
  • one-line install for Linux/macOS and Windows
  • pinned version install
  • PATH setup
  • checksum/release archive behavior
  • optional dp alias policy and opt-in commands
  • collision behavior when another dp command exists
• Quickstart page
  • datapan init --json
  • datapan status --json
  • datapan ready --limit 10 --json
  • datapan try ... --json
  • datapan kit ... --json
• Command reference page
  • stable command families grouped by workflow
  • registry install/search/call/export/codegen/cache/release evidence commands
  • machine-readable --json contract notes
• Troubleshooting page
  • missing PATH
  • missing data.go.kr key
  • registry install failures
  • GitHub release download/rate-limit failures
  • dp alias skipped due command conflict

Acceptance Criteria

• Published docs explain that datapan is the canonical command and dp is an optional alias.
• Install docs include explicit opt-in examples: DATAPAN_INSTALL_DP=1 and -InstallAlias.
• Non-interactive installer behavior is documented: no prompt, alias skipped unless explicitly enabled.
• Docs include copy-paste first-run commands and expected verification commands.
• README links to the published install and quickstart docs once the docs surface exists.
• The docs structure is intentionally similar to Gira's docs-site split: install, quickstart, command reference, troubleshooting.

[StatPan]

Implementation note: alias management now lives in the Go CLI as datapan alias status [dp], datapan alias install dp, and datapan alias remove dp. Install docs should present the Go command as the normal post-install path, with DATAPAN_INSTALL_DP=1 and PowerShell -InstallAlias documented as bootstrap-time shortcuts only.

[StatPan]

Correction: installers should not expose alias install flags or interactive alias behavior. The documented and supported path should be: install the canonical datapan binary first, then run datapan alias install dp if the user wants the optional shortcut. Bootstrap scripts stay binary-only.