Improve point_cloud_library clean_html

[ardiya]

This commit tries to improve the clean_html with 2 updates:

1. Improve table appearance by removing unnecessary gap between table

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After:

2. Improve code appearance

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After:

cc: @kunaltyagi <- PCL maintainer who requested this PR

[xjkdev]

Hi! Because PCL's documents are also generated by doxygen, there is a same issue that the space indents in code fragments were removed. And I have solved this problem by writing filter codes toparse function in UrlScrapper instead.

And I believe it can be better if you do the same. please check #1717 and lib/docs/scrapers/eigen3.rb .