[Track D] Docs consolidation — fold Studio + Guard docs into root docs/

# [Track D] Docs consolidation — fold Studio + Guard docs into root `docs/`

_Sub-task of #2600 — Track D._

## Goal

Consolidate Studio + Guard docs into root `docs/`, retire nested Docusaurus sites, and add new docs for desktop install + Hub launchers.

## Scope (D1 from #2600)

- [ ] **D1.** Finish consolidating Studio + Guard docs into root `docs/`:
- Studio user docs already live there — fold A2-retained docs alongside
- Retire nested Docusaurus sites under `calm-suite/calm-studio/docs` and any `calm-guard/docs` Docusaurus
- Add **desktop install/download docs** for CALM Studio (post-C2)
- Add **"Open in…" from Hub docs** (post-C7)
- Add **"pack authoring" guide** (post-C8)
- Add **"workspaces + Studio offline/sync" guide** (post-C5)
- **Migration guide** for directory move + package renames (`@calmstudio/mcp` → `@finos/calm-mcp` etc.), incl. MCP client config snippets + 1-to-1 map from old Java MCP tools to new `hub_*` tools
- Updated `README.md` / CODEOWNERS / `docs.calm.finos.org` entries for new top-level `calm-guard` + `calm-studio`

## Why this matters

- Single docs surface across the platform (no nested Docusaurus sites to maintain)
- Migration path for external users picking up the renames
- Operational docs for new capabilities (desktop install, launchers, pack authoring)

## Out of scope

- Implementing Studio/Guard/MCP changes themselves (covered by Tracks A/B/C)
- New marketing/landing content beyond consolidation

## References

- Parent: #2600
- Depends on: Track A #2649 (A2 doc retention decisions, A3 path moves), Track B #2650 (B3 MCP rename for migration guide), Track C #2651 (C2/C5/C7/C8 new feature docs)
- Touches: `docs/`, `README.md`, CODEOWNERS, `docs.calm.finos.org`