Add more docs for....

1. Requirements Gotchas
2. Optional values - input (try/can)
3. Optional values - output (merge)
4. Publishing set up for plugins in MAINTAINERS.md
5. Implicit schema derivation in HCL gotchas (e.g. `foo ? "bar" : true` is not allowed)

Start by locating the existing documentation for requirements, optional values, HCL schema derivation, and plugin publishing, then review MAINTAINERS.md for the publishing setup. Compare the listed gotchas with the current docs and examples; done means each requested topic is clearly documented, including the invalid conditional example.

Written by the indexing model from the issue text.