Generate consumable documentation from the spec

Spec files currently contain information about how each file and field should be used, with descriptions, examples, accepted values and so on. But these are source files that may be renamed, moved or removed, breaking documentation builds that depend on them.

Generate some intermediary documentation files in a well-known location (like `docs/spec.asciidoc`) that other projects may refer to.

And revert https://github.com/elastic/package-spec/pull/401.