feat: add squad splitting best practices documentation

[selena-vapi]

Description

• Add new documentation page fern/squads/splitting-best-practices.mdx with comprehensive best practices for designing and configuring multi-assistant squads
• Add navigation entry in fern/docs.yml positioned between "Handoff tool" and "Examples" in the Squads section
• Content covers: when to use squads vs single assistants, identifying functional boundaries, architecture patterns (hub-and-spoke, linear pipeline, language router), handoff configuration with model-specific patterns, context engineering options, variable extraction, system prompt best practices with template, all four transfer modes, configuration tips (assistant overrides, tools:append, member overrides, silent transfers), seven common pitfalls with remediation, and card links to all existing squad example pages
• All 5 JSON code blocks validated as syntactically correct; all internal cross-reference links verified against existing files
• 374 lines of MDX content + 3 lines YAML change = 377 total lines (within 500-line limit)
• Closes DEVREL-551

Testing Steps

• Run the app locally using fern docs dev or navigate to preview deployment
• Ensure that the changed pages and code snippets work
• Navigate to the Squads section and confirm "Splitting best practices" appears between "Handoff tool" and "Examples"
• Verify all internal links on the new page resolve correctly (links to /squads, /squads/handoff, /squads/silent-handoffs, /squads/examples/*, /api-reference/squads/create)
• Verify anchor links to handoff.mdx sections work (#multiple-destinations, #variable-extraction, #system-prompt-best-practices)