Final call: Link to related rules in background

Hey folks,
This pull request adds links between closely related roles to the
background section. This should help common questions of "why is X excluded
from the rule?". This final call lasts for 1 week, ending on October 22nd.
If you have any comments, please leave a review on the pull request:

https://github.com/act-rules/act-rules.github.io/pull/1483

-- 
*Wilco Fiers*
Axe-core product owner - Co-facilitator WCAG-ACT - Chair ACT-R


Join me at axe-con <http://deque.com/axe-con> 2021: a free digital
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