docs: simplify README around three processes - #4591

Description

Make the README a practical starting point rather than a combined tutorial, reference, and project history. Reduce it from 492 to ~195 lines, with one shared installation/initialization and three independent entry points: Spec-Driven Development, bug fixing, and idea assessment. Process steps are described as skills invoked in the coding agent's chat, with the terminal reserved for CLI installation and project/extension setup.

• Keep runnable step sequences, Copilot's default skills syntax, the SDD convergence loop, and explicit opt-in installation for bug and assess.
• Add beginner bug-fixing and assessment walkthroughs, an assessment command reference, and a customization guide.
• Consolidate detailed command tables and examples, CI initialization guidance, video, anniversary history, community publishing guidance, and dogfooding details into the docs and contributor guide. Reuse existing installation, upgrade, philosophy, and component references rather than duplicating them.
• Align the docs homepage, reference overview, existing-project guidance, and Chinese README. Retain important README entry anchors and existing SDD walkthrough anchors.
• Keep acknowledgements in the history page instead of repeating them in the READMEs; retain the license links.
• Place all three process links under the existing Getting Started navigation group to avoid a reproducible 1024px navbar overflow observed with a separate top-level menu.

Testing

Documentation-only change; no CLI implementation, templates, extension manifests, or catalogs changed.

• Built the site with DocFX 2.78.5: zero errors, with the same two pre-existing warnings about unsupported ../ glob patterns in docs/docfx.json.
• Checked all changed Markdown files for rendered destinations and newly broken links/anchors; verified all process pages are reachable from the homepage and navigation.
• Checked English/Chinese README command-block parity, one shared install/init, all process steps, and opt-in extension setup.
• Compared rendered upstream and revised homepages at 1440, 1024, 896, 768, 390, and 320px; the final navigation adds no horizontal overflow relative to upstream.

The template's CLI checks are not applicable to this documentation-only change.

AI Disclosure

• I did use AI assistance (describe below)

GitHub Copilot (models: gpt-6-astra and Claude Opus 4.8), acting on behalf of @mnriem, authored the documentation changes, coordinated translation and an independent review, ran the documentation/browser checks, and fixed the review findings. The human requested and directed the scope; agent execution was autonomous, not human line-by-line authorship. The commits include Assisted-by and Co-authored-by trailers.