Validate docc content as part of background indexing

**Is your feature request related to a problem? Please describe.** I write a lot of docc content in our packages (both doc comments and docc articles), and often I make a typo in a reference. Today, I don't catch these bugs until I open a PR, and our CI that validates the docc content catches it. Then I need to fix it, push, which restarts all the CI jobs, etc. **Describe the solution you'd like** It'd be great if VS Code validated the docc content just like we get build warnings/errors using background indexing as we type, without needing to explicitly trigger anything. It's okay if it's an opt-in setting, but I'd be very interested in this. Basically the functionality that you get when you use the swift-docc-plugin and you ask it to analyze the content - it both validated doc comments, as well as articles in your docc bundle. I'd want normal warnings to show up next to code warnings, in the VS Code sidebar. **Describe alternatives you've considered** Manually triggering docc validation - I do that today, and it's quite tedious. **Additional context**