[Docs] Add "Related" section to References

[adevade]

It would be nice if below the info of the Modifier, Tag and Variable, there was a "Related" section with similar functions.

For example, if I'm viewing the days_ago modifier, between the content and the "Docs Feedback" widget there could be a "Related modifiers" listing hours_ago, minutes_ago, months_ago, etc...

I can guess and find it in the sidebar, but it's not always as similar naming as in the example above.

[jackmcdade]

Good Idea! I see you opened the issue on the docs repo so anyone following can track this there. statamic/docs#714