[Docs] Make the README a concise documentation entry point

[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

Developers need to understand the skeleton's purpose and find their next task quickly. Keep README.md as a short landing page; detailed installation alternatives and command references belong in the guides.

Proposed structure

1. What this is: a block-theme skeleton, its intended users, and its relationship to wp-framework and wp-tooling.
2. What is included: a brief summary of initialization, supplied examples, asset builds, optional features, checks, and AI tooling.
3. Get started: one prominent link to the verified Getting Started path and a short note identifying the currently supported release/branch.
4. Choose your next task: initialize, run locally, explore included functionality, generate a feature, or extend it manually.
5. Contributing and license: links to maintainer guidance.

Editorial requirements

• Do not repeat the full installation sequence or every command.
• Link the feature summary to docs/features.md; detailed framework classes remain upstream.
• Label any temporary v2 installation note and remove it when no longer applicable.
• Link only to capabilities actually retained and usable in the resulting project.

Acceptance criteria

• A new developer can identify the purpose, included functionality, and next guide in under one minute.
• Getting Started is the primary entry point.
• Task links cover setup, local development, scaffolding, and manual extension.
• No duplicate setup tutorial or exhaustive command reference is introduced.
• All links resolve for the supported skeleton and documented downstream state.