doc: Consolidate Python and pip documentation

The documentation that describes how Python and pip interact with the OS
when installing packages used to be under a common section, and was
moved to the west bootstrap one later on. Since this information is
required early on (for example on Linux when installing CMake via pip3),
move the info to its own section and link to it from others.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>