docs: update skills path precedence

[enyst]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

• Document .agents/skills as the repo-level skill directory with legacy .openhands/skills.
• Add project-level skill loading precedence notes.
• Update keyword example and architecture location reference.

[openhands-ai]

Looks like there are a few issues preventing this PR from being merged!

• GitHub Actions are failing:
  • .github/workflows/sync-docs-code-blocks.yml
  • .github/workflows/sync-agent-sdk-openapi.yml

If you'd like me to help, just leave a comment, like

@OpenHands please fix the failing actions on PR #307 at branch `docs/agents-skills-precedence`

Feel free to include any additional details that might help me get this PR into a better state.

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