Document NavFn max_cycles_factor

[R0xy42]

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses	ros-navigation/navigation2#6308
Does this PR contain AI-generated software?	No; this is a documentation-only change. The wording was drafted with AI assistance and reviewed by me.

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Description of contribution in a few bullet points

• Document the new integer max_cycles_factor parameter introduced by Make NavFn path extraction cycle limit configurable navigation2#6308.
• Explain how the path extraction cycle limit is calculated from the largest costmap dimension.
• Add a concrete 500 by 400 cell costmap example using the default factor of 4.
• Add max_cycles_factor: 4 to the NavFn YAML configuration example.

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