Improve consistiency of marking external links with an icon

[zackkrida]

Problem

Identified by @seancolsen in #4126 (comment).

Mathesar includes a number of "external" links; for example, links out to the documentation site. Some of these use an icon to indicate that you will be leaving Mathesar. Others do not.

Proposed solution

Move the "external hyperlink" icon into the DocsLink component so that it appears everywhere. This would require visual inspection to make sure it looks correct in all the different usages of the DocsLink component.

Additional context

[kgodey]

@zackkrida Can you mark this with either a ready or some sort of needs: label?