docs(start_planner): add freespace pull out docs

[kosuke55]

Description

add freespace pull out docs

Tests performed

Not applicable.

Effects on system behavior

Not applicable.

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In-review checklist for the PR reviewers

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Post-review checklist for the PR author

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[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (0cd4f62) 14.86% compared to head (1099113) 14.86%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5161   +/-   ##
=======================================
  Coverage   14.86%   14.86%           
=======================================
  Files        1627     1627           
  Lines      112571   112571           
  Branches    34766    34766           
=======================================
  Hits        16739    16739           
  Misses      77076    77076           
  Partials    18756    18756           

Flag	Coverage Δ		*Carryforward flag
differential	12.55% <ø> (?)
total	14.86% <ø> (ø)		Carriedforward from 0cd4f62

*This pull request uses carry forward flags. Click here to find out more.

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