[DOC] Fix broken links

CONTRIBUTING.md

doc/README

doc/about.rst

doc/datasets/twenty_newsgroups.rst

doc/developers/advanced_installation.rst

doc/developers/contributing.rst

doc/developers/performance.rst

doc/developers/utilities.rst

doc/install.rst

doc/modules/clustering.rst

doc/modules/computational_performance.rst

doc/modules/cross_validation.rst

doc/modules/decomposition.rst

doc/modules/density.rst

doc/modules/ensemble.rst

doc/modules/feature_extraction.rst

doc/modules/feature_selection.rst

doc/modules/gaussian_process.rst

doc/modules/kernel_approximation.rst

doc/modules/label_propagation.rst