[Fleet] Use DocLinksService for doc links

[jen-huang]

In Fleet, we have areas where we link to documentation without specifying version in the path (using /current/ instead). This will be a problem because users will be sent to the latest documentation instead of the version they are running.

We also have places where we use short URLs as a placeholder before docs are ready. This will run into the same problem with the user's version not being respected.

Instead, let's replace both kind of usages with real doc links (instead of short URLs) that are versioned, via DocLinksService.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[dedemorton]

We should also take this opportunity to refine some of the links that point to the guide. In many cases, we provide a generic link to the Fleet User Guide (mainly because the topic needed to support the UI didn't exist at the time the UI was created). We might not have coverage in all cases yet, but I think we're closer!