docs: fill VS Code Marketplace listing and surface install paths

[arnaudlh]

Summary

The published VS Code Marketplace listing for Git-ApeTeam.git-ape had an empty Overview tab because the VSIX never contained a README. This PR fills the listing and surfaces the new install path across the project docs.

What changed

Fill the Marketplace listing

• extension/README.md (new) — rendered as the Marketplace Overview tab and as the VS Code Extension details panel. Includes:
  • Project intro and warning callout.
  • One-click install badges (Marketplace, vscode:extension/..., vscode-insiders:extension/...).
  • Two install options: (A) one-click from the listing, (B) chat.plugins.marketplaces setting for users who also want `ape-context` or latest-from-GitHub updates.
  • Tables enumerating the 8 agents and 13 skills that ship in the VSIX.
  • Architecture diagram, docs links, contributing/security/license section.
• extension/CHANGELOG.md (new) — stub pointing to GitHub Releases and the docs site changelog.
• No build-workflow changes needed: `extension/.vscodeignore` already passes `README.md` and `CHANGELOG.md` through to the VSIX (verified with `vsce ls`).

Surface the Marketplace install path

• README.md — header now lists both the docs site and the Marketplace listing. Install section gains a new Option A: VS Code Marketplace (one-click) with shields.io badges; previous options renumber to B/C/D.
• website/docs/getting-started/installation.md — comparison table led by the new Marketplace option. New `## Option A: VS Code Marketplace (one-click)` section mirrors the README. Existing slugs (`#vscode-plugin`, `#cli-plugin`, `#local-dev`) preserved for backward compatibility.
• website/docs/intro.md — adds Marketplace install badges to Quick Start.
• website/docs/getting-started/vscode-vs-cli.md — "Marketplace install" parity row now links directly to the listing; workspace-recommendation section clarifies the auto-install path is the agent-plugin route and points to the Marketplace as a fallback when `chat.plugins.enabled` is disabled.
• website/docs/getting-started/onboarding.md — agent-plugin requirement info box now distinguishes the two install paths.

Correct prerequisite framing

The Marketplace VSIX installs as a regular VS Code extension, so it does not require `chat.plugins.enabled`. Only the agent-plugin install paths (`chat.plugins.marketplaces`, Chat: Install Plugin From Source, `copilot plugin install`) are gated by that org-level setting. All docs updated to reflect this.

Verification

• `vsce ls` (run from `extension/`) confirms `README.md` and `CHANGELOG.md` are in the VSIX payload.
• Docusaurus production build is clean: `DOCUSAURUS_BASE_URL=/ npm run --prefix website build` ➜ `[SUCCESS] Generated static files in "build".` Only the pre-existing `vscode-languageserver-types` webpack warning remains.
• `get_errors` reports no markdown/MDX errors on any of the changed files.

Diff stats

```
README.md | 21 +++++++++++++++------
extension/CHANGELOG.md | (new)
extension/README.md | (new)
website/docs/getting-started/installation.md | 24 ++++++++++++++++++-----
website/docs/getting-started/onboarding.md | 2 +-
website/docs/getting-started/vscode-vs-cli.md | 6 ++++--
website/docs/intro.md | 11 ++++++++++-
```

Out of scope

• No build/workflow changes.
• No agent or skill content changes (only the README enumeration of them).
• No source-code changes to the plugin manifest beyond what the published v0.0.2 already contains.

Follow-ups (separate PRs)

• The Marketplace Features tab still shows empty Name/Description columns for `chatAgents`/`chatSkills`. The contribution-point schema only accepts `path` and `when` — VS Code reads name/description from the agent/SKILL frontmatter at install time. Filling those Marketplace columns is a Marketplace-side rendering limitation, not a packaging fix.

[suuus]

Was pressured to accept this one.