Improve linter output formatting for conda-forge.yml

Some notes [about the linter]:

it doesn't tell which file this is all about
- it could include a link to that exact file in the PR branch, such that a maintainer could click to edit
as the output is for humans, it seems like rendering title, description, etc. as Markdown would be valuable.
- however, as it was written (or dumped to) JSON Schema, it's would possible to generate a canonical link to the generated documentation for that schema item, once available
- similarly, the raw repr that gets dumped could be a table
- or, indeed, the whole thing could be a table
- it may be worth putting these inside details tags

Now all we gotta do is schematize the other user-edited .yml :)

conda-forge / conda-smithy