Enhance README structure and update devspec installation documentation

[gvreddy04]

This pull request significantly restructures and clarifies the README.md and supporting documentation to make setup, workflow, and operational guidance more concise and accessible. The main improvements include streamlining installation and workflow instructions, reorganizing documentation for easier navigation, and separating out key reference material into dedicated sections and tables.

Key changes include:

Documentation and Workflow Simplification:

• The README.md installation and workflow instructions are now much more concise, with step-by-step guidance condensed and clarified. Redundant or overly detailed sections have been removed or summarized, and references to full guides are provided instead.
• The core workflow is now explained with a clear two-layer model (Foundation and Work items), each with direct links to full guides, replacing the previous inlined command and flowchart sections.

Restructuring and Thematic Organization:

• Setup options and repository layout are now presented as tables for quick reference, with new sections for "Setup Options," "Core Workflow," and "More Documentation."
• The command reference, adapter/tool support, and manual copy fallback are now summarized with links to dedicated documentation, reducing clutter and improving maintainability.

Navigation and Accessibility Improvements:

• The table of contents in docs/how-to/README.md is updated to include new sections such as "Repository Layout," "Operating Rules," and "Recommended Adoption," improving navigation.
• A new "Operating Rules" section is added to both the main and usage documentation, summarizing best practices for using and adapting devspec.

Clarification of Adapter and Tool Support:

• The AI tool support matrix is clarified, with each tool's setup and integration notes presented in a simplified table, and references to command invocation and compatibility documentation.

Reference Material Consolidation:

• Repository layout and file purpose are now described in a dedicated section and table, with clear guidance on what should and should not be copied or modified in target repositories.

Overall, these changes make the documentation easier to follow, reduce duplication, and help new users and teams get started with devspec more efficiently.

• Documentation and workflow simplification: streamlined installation, workflow, and core usage instructions in README.md, with redundant sections removed and references to detailed guides added.
• Thematic restructuring: setup options, repository layout, and tool support are now organized in tables and dedicated sections for clarity and quick reference.
• Improved navigation: updated table of contents and added new sections such as "Operating Rules" and "Recommended Adoption" to enhance usability.
• Clarified adapter/tool support: simplified tool setup notes and command invocation guidance for each supported AI coding agent.
• Consolidated reference material: repository layout, operating rules, and best practices are now clearly summarized in their own sections.