[#1557] Add support for building experimental documentation.


Problem description
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This branch add support for building documentation using experimental sections.


Changes description
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As a side fix, the which command was updates so that extra paths are checked first. I was having problems with the build system since my system also contain files conflicting with the versions from extra paths...and which was returning the old version from my system.

There are no tests for the code. To check the functionality please check https://github.com/chevah/server/pull/210


How to try and test the changes
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reviewers @alibotean 

This is just to let you know about the low level changes.