docs: replace removed make targets in local demo guides #135

Part of #85.

The TechDocs entry page and Plausible tracking guide still direct visitors to removed make targets, including login and deployment commands that have no current equivalent. Bring these two local demo guides into line with the existing mise tasks.

Acceptance criteria

• The entry page includes a working repository-root local quickstart, the frontend/backend URLs and the guest demo path.
• The task table lists only existing mise tasks and links to the current demo baseline and development guide.
• The analytics guide uses the existing mise tasks for local startup and teardown, describes the optional Docker requirement, and documents the actual frontend Plausible settings.
• Verify install, lint and tests; cross-check documented task names against mise.toml. This is documentation only, with the changed documents serving as proof.