doc: posix arch: Refactor arch docs into own page

Refactor the POSIX arch documentation out of the
native_posix page into its own page.
Originally native_posix was the only POSIX arch based
board, but this has not been the case anymore for several
years.
The architecture part of the documentation being in the
native_posix doc has created uneeded confusion over time.
Both due to this, and as preparation for much more
documentation for the bsim boards, refactor it out.

This is mostly a refactoring exercise, with only very
minor changes in the text to keep it consistant with
the move.

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>