Improve contrast on syntax highlighting

[Katy DeCorah]

There are a few tokens in our syntax highlighting CSS that need contrast improvements:

• span.token.namespace - Prism applies opacity: 0.7 to this class. If we remove the opacity (or set opacity: 1 to override) then the color contrast should pass. Example page where namespace contrast is too low.
• code when it's a link (token) - We should add a class to docs-prose.css to override the color value of a a code to a text color with a higher contrast (maybe #3d5ef2) - Example page where code link contrast is too low.

Let's keep this issue updated as we come across more.

[sentry-io]

Sentry issue: DOCS-LIGHTHOUSE-1N