Iterates on https://github.com/Unleash/unleash/pull/5762
While on the previous PR we would always open markdown links on a new
tab, we still want to navigate on the same tab when a relative link is
specified.
This adds a new `Markdown` common component with this logic by default,
which should make things a lot simpler and easier to maintain. The logic
that was followed is similar to the existing internal/external links
logic in our banners.
