I absolutely agree that this is an issue - and I run into it constantly myself, as I spend a LOT of time working with documentation on projects. Not having the equivalent of the docc-plugin immediately available is incredibly frustrating, and it often becomes extremely annoying to projects who, quite reasonably, do not want an additional dependency on their project that slows down builds when they don't need the docs to be a part of that process.
The fact that it is extra work to get easy docs is a massive friction point in the overall ecosystem that needs to be eliminated.
The how is a really good question. A year or so back I dug down this path, thinking that maybe there'd be a good option to have a set of Package Manager plugins that were "just available by default" - or that could be activated or installed by developers - specifically to provide command plugins like generate-documentation. That said, after making the pitch I didn't have the time to follow up and provide any prototypes or start implementing anything there.
Somewhere along the same timeframes, the other command plugin that I thought was really needed - swift format - got itself a bit more embedded in the toolchain directly, although the swift format command is still a "driver" command, and not the super-developer friendly option up front, which has been talked about elsewhere - orthogonal to this.
I'll also note that Dave and Sven, in providing packages for Swift Package Index, have to deal with the "is it added or not" scenario for the documentation builds as well, so anything we enable here would greatly ease being able to build, preview, and host documentation content.
I've also been wanting to see better integration with the swift-argument-parser tooling - the generation of markdown is nice, but it could be a lot easier and better, although that also needs some time and attention to enable.
And this is perhaps a bit more about about DocC and how it works than this pitch, I'd very much like to see a way to include content from the repository for some more edge use cases, particularly when it comes to Swift and other language interop (embedded, swift/java, swift/c or swift/c++). Today there's snippets - which are still tricky to use, and only work for Swift code. It would be very useful to be able to reference other content from the repository in some fashion that you could "include" into the DocC markdown content - be those C source code, README files, JSON snippets, and so on.
My real question is how are you seeing expanding this? A new and separate tool into the toolchain, potentially starting with swift-docc-plugin and its existing coordination with Swift Package Manager? Changing up the API exposed from SwiftPM to allow this in some other context? Enabling additional commands that don't require a Swift package dependency, but that can be included or downloaded into a toolchain or workspace?
I'm game, and I'd very much like to help and make this a reality, but I'm a bit stymied by the how at the moment.