Developer documentation gives bad advice

The developer documentation is currently fragmented and gives bad advice for contributors. The documentation in CONTRIBUTING.md links to INSTALL instructions which include none of the DEBUG build flag instructions which would normally be the default for all contributors/programmers who wanted to build inkscape.

I believe the documentation refactoring had erroneously merged generic build instructions with contributing build instructions, thinking that they are the same thing.

Debug flags are documented in a file called general_advanced but debug is neither advanced nor general. It's the DEFAULT build instruction for new contributors.

The contributing build instructions should be reverted and a better narrative should be created for the various flags we wish to document in general_advanced.