Update .gov content guide linking recommendations to align with accessibility best practices

Issue description

Some of the links on get.gov open in a new tab while others don't. W3C recommends opening new windows and tabs from a link only when necessary since it can be disorienting for users and difficult to navigate back to the previous page (source).

We should reevaluate our linking strategy as a team and update the .gov content guide as appropriate.

Acceptance criteria

[ ] W3C's recommendation has been shared with the team (ex. in product + design huddle) for alignment on guidelines related to opening links in a new tab vs not
[ ] A recommendation based on team feedback has been documented and shared with Katherine and Cameron for approval
[ ] The .gov content style guide is updated as needed and shared with the team
[ ] Create a dev ticket to implement the changes.

Additional context

No response

Links to other issues

Relates to #1917 Additional context can be found in https://github.com/cisagov/get.gov/pull/280

Prioritization Comments

WCAG Conformance Level: 2.1 AA WCAG Standard that applies: 3.2.5 (Change on Request) Critical Failure if Not Met?: No Prioritization: Normal/Should Do Reason:

cisagov / manage.get.gov