Move the Tuning Cores section to nix.conf

[fricklerhandwerk]

Problem

The contents of Tuning Cores is regular reference documentation that is hard to find outside where one would expect the parameter documentation.

Proposal

Move the information to the documentation of cores and link to it from max-jobs; remove the page.

Important
add a redirect on nixos.org, otherwise existing links will break

Priorities

Add 👍 to issues you find important.