Add rule proposal to docs

[fisker]

Unlike the implementation and changes, they can easily find by checking commit history.

The proposal link can only find by search (diffcult since rule names changes during time), or linked in the initial implement PR.

People sometimes curious why the rule exists (especially when unexpected problem occurred), add the proposal link may help people understand better what the problem rules want solve and decide bug should be reported.

Useful links for huge changes can also listed.

[sindresorhus]

I usually just go to the rule docs and click "history" to find the commit that introduced it. I guess it can be useful on some cases. Should we always do it or only when the rule proposal issue has something useful? Sometimes the rule proposal issue is very minimal and all the useful background and decisions are in the pull request discussions.

[fregante]

why the rule exists

This should 100% be part of the rule description. What's covered and what's not is generally part of the examples as well.

[fregante]

Previously reported:

• Include reason to use rule in documentation #1752