Various doc updates removing old TCL build option, build instructions link, and old macOS names (#5419)

* DOC: Remove mention of TCL support build option

This build option was removed in 9a9c2b19.

* DOC: Replace build instructions link

All build instructions have been moved over to the Slicer readthedocs.

* DOC: Use correct case and name for macOS

macOS Sierra (10.12) was the first version that adopted the new macOS name (previously known as OS X and Mac OS x).