Visited links are colored purple in the docs

[asmeurer]

With the most recent update of Furo, visited links are now purple in the docs (this is currently only the case in the dev docs)

When we first set up the Furo theme, we decided not to use a visited link color #23159. The purple color that Furo uses by default does not seem to have any color contrast issues. The main concern is whether it clashes too much stylistically with the green color theme. We should either:

• Find a different color that goes better with green and use that. If we do this, be sure to check the color contrast both for the background and surrounding text (see https://webaim.org/resources/linkcontrastchecker/).
• Remove visited link coloring entirely.
• Keep things as they are, and close this issue.

I don't feel particularly strongly about any of these options.