Make a plan for linking external sites going forward

[JonellaCulmer]

What we're after:
We treat links to external sites differently in different places. We need to define a "policy" and document (including any exceptions) under what conditions sites are linked and how.

Related issues

#4335 Review and update Content Guide to capture modifications we are making
#4235 Make a plan for footnotes going forward

Completion criteria

• Define when, how and under what circumstances we display the destination or URL of an external site.

Example: www.fec.gov or Federal Election Commission