[Docs] Add one verified route to a running theme

[divyarajmasani]

Parent epic: #768 — Documentation epic

Base branch: theme-elementary-v2. Cover its functionality and subsequent changes based on it. Verify against the latest relevant v2 implementation and record the tested commit in the documentation PR.

Problem and scope

The current Quick Start mixes pilot workarounds, setup alternatives, and feature tutorials. Replace or supersede docs/quick-start-guide.md with docs/getting-started.md, ending at a working local theme.

Proposed structure

1. Outcome and starting context: fresh standalone project versus a theme added to an existing WordPress/VIP repository.
2. Prerequisites: supported versions and short verification commands; distinguish requirements for the recommended wp-env route from other existing local environments.
3. Acquire the skeleton: one tested installation path for the supported revision. Keep temporary pilot exceptions behind a link.
4. Install dependencies: explain actual Composer/npm behavior without duplicate installation steps.
5. Personalize: use the CLI wizard or the supported AI alternative; explicitly state that both are not required.
6. Start and view the theme: minimal environment, activation, and build steps, with an expected URL/result.
7. Next task: links to daily development, included functionality, and extension examples.

Context and checkpoints

• State the working directory for commands.
• For an existing repository, show the destination theme directory and distinguish theme-local scripts/instructions from root-level project tooling. Link to the relevant project/VIP guide; do not imply init is safe at the wp-content root.
• After personalization, review identity and cleanup changes; suggest a baseline commit before feature work.
• Confirm the theme loads and a source edit appears. Detailed troubleshooting belongs in the relevant guide.
• Update links from the old Quick Start location.

Acceptance criteria

• The recommended route is tested in a fresh disposable project.
• Developers can reach a running theme without consulting framework source.
• Existing WordPress/VIP users can identify the correct destination and working directory.
• CLI and AI initialization are clearly alternatives.
• Personalization and first-render checkpoints show what success looks like.
• Pilot procedures and detailed extension tutorials do not interrupt the main path.