boards: mps2_an521: Update outdated links

This commit updates all outdated links in the mps2_an521 board
documentation.

Some links updated here were not completely broken (redirects worked),
but they did not redirect to the correct page in the documentation, so
they were also updated to point to the correct page.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>