[Enhance] Add visualization example and instruction to FCOS3D cfg page

[Wuziyi616]

See #542 (comment).

[codecov]

Codecov Report

Merging #582 (c55e4a9) into master (926beda) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #582   +/-   ##
=======================================
  Coverage   50.82%   50.82%           
=======================================
  Files         197      197           
  Lines       15050    15050           
  Branches     2451     2451           
=======================================
  Hits         7649     7649           
  Misses       6891     6891           
  Partials      510      510           

Flag	Coverage Δ
unittests	50.82% <ø> (ø)

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