Stardoc for targets

Sometimes the target's name and rule kind don't provide much detail on its purpose. When should the target be used? Is it runnable, testable, or just buildable? Maybe it's meant only to be used as part of a larger dependency?

This kind of information along with details that come from the rule could be a good source of documentation for a package, especially for newcomers. Imagine looking through a BUILD.bazel file with a bunch of sh_binary() targets. This could seem very opaque without some context.

No source file, test, or entry point is named. Start by reviewing how Stardoc currently documents rules and targets, then define which target-purpose and rule-derived details should appear in package documentation. Done means generated documentation makes target usage and buildability clear, including for targets such as sh_binary().

Written by the indexing model from the issue text.