Fix docs on Designer UI by using rel paths

[jfomhover]

The page on designer ui didn't show images because they used / as absolute path. This PR fixes that.

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Unit Test Results for Build

  1 files  ±0    1 suites  ±0   52s :stopwatch: -6s 97 tests ±0  97 :heavy_check_mark: ±0  0 :zzz: ±0  0 :x: ±0 

Results for commit 585ff45f. ± Comparison against base commit 446586eb.

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Package	Line Rate	Branch Rate	Complexity
common	88%	0%	0
pipelines.azureml	83%	0%	0
scripts	100%	0%	0
scripts.data_processing.generate_data	93%	0%	0
scripts.data_processing.lightgbm_data2bin	95%	0%	0
scripts.data_processing.partition_data	92%	0%	0
scripts.inferencing.custom_win_cli	94%	0%	0
scripts.inferencing.lightgbm_c_api	75%	0%	0
scripts.inferencing.lightgbm_python	95%	0%	0
scripts.inferencing.treelite_python	94%	0%	0
scripts.model_transformation.treelite_compile	92%	0%	0
scripts.sample	93%	0%	0
scripts.training.lightgbm_python	80%	0%	0
Summary	87% (1516 / 1733)	0% (0 / 0)	0