docs: improve README navigation and skill catalog discoverability

[rosspeili]

Improves README discoverability and documentation navigation without duplicating usage-guide content. Adds an early Skill library link under Mission, syncs the architecture tree with the current repo layout, compacts the Documentation section into a scannable table, and adds provider links after the Gemini example. Cross-links the skill catalog back to the project README. Keeps core install provider-agnostic (pip install -e .); scopes the [gemini] extra to the Gemini example section only.

Fixes #72

Description

README-only documentation update for #72:

• Mission — compact Skill library link with category names
• Architecture — minimal tree sync (card.json, docs/usage/, provider scripts in examples/)
• Quick Start — core install stays pip install -e .; Gemini extra noted only under §4 Usage Example
• Documentation — compact table with inline provider links instead of a long bullet list
• docs/skills/README.md — link back to project README; brief skillware list hint

No skill logic, framework code, or dependency changes.

Type of Change (Matches Issue Templates)

• Skill Proposal: New Skill (Contains manifest.yaml, skill.py, and instructions.md)
• Bug Report Fix: Non-breaking change which fixes an execution error or framework bug
• Doc Fix: Documentation Update
• Framework Feature / RFC Updates: Core Framework Update (Changes to base_skill.py, loader.py, etc.)

Checklist (all PRs)

• My code follows the Agent Code of Conduct.
• I have run python -m flake8 . and pytest tests/ locally (or the subset relevant to this change).

New or updated skill

Skipped — documentation-only PR.

Constitution & Safety

Not applicable — no skill or runtime changes.

Related Issues

Fixes #72