docs: add Fia Signals integration docs (#4690)

[yuweuii]

Summary

This PR implements issue #4690.

• Scope: Tool integration: Fía Signals crypto intelligence (pip install fia-signals-tools)
• Source branch: yuweuii:codex/issue-4690
• Commit: e9541ec

Linked Issue

Closes #4690

---

Note

Low Risk
Low risk documentation-only change; the main potential impact is CI failures if future docs navigation structure or hrefs change and break the new docs-linking test.

Overview
Adds a new integration docs page for Fía Signals Tools (fiasignalstools.mdx) including install, available functions, and a CrewAI usage snippet.

Registers the new page in docs.json navigation and links it from both integration overview pages, and adds a small pytest (test_fia_signals_integration_docs.py) to assert the page exists and is referenced from nav/overviews.

^{Written by Cursor Bugbot for commit 1de81e9. This will update automatically on new commits. Configure here.}

[greysonlalonde]

Hi! We are generally not accepting any external tool additions at the moment. Please see https://docs.crewai.com/en/guides/tools/publish-custom-tools