Minor changes to www.tensorflow.org/install/install_sources

1. Remove mock from extra packages to install for testing since
   it is already required to be installed for the build.
2. The instruction to build the build_pip_package script
   is difficult to notice as it gets lost with
   the Note in the GPU support section above it.  As such,
   add a subsection title for it.
RELNOTES: n/a

PiperOrigin-RevId: 209159059