Minor update to the background section

[WilcoFiers]

The ACT-R community group is considering updating the background section. This was discussed yesterday on the CG call. The proposal is to add two headings to the background section "Bibliography" and "Related rules". This should keep the list of links a little better organised. Find the details here:

act-rules/act-rules.github.io#1531 (comment)

The community group would like the opinion of the task force, before we adopt this change.