feat: parse into a Document that keeps order and comments

Assisted-by: DeepSeek V4.1 Flash

CHANGELOG.md

@@ -28,6 +28,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   at most `threshold` bytes is written as an inline table instead of a header
   section, which shortens a document of small tables. An array of tables keeps
   its header form, because its inline form would re-parse as a value array.
+- `Document` and `ParseMap`: `Parse` now returns a `*Document`, which holds the
+  values together with the key order, whether a table was written as an inline
+  table or under a header, and the comments, with `Keys`, `Entries`, `Get`,
+  `Comments` and `SetComments` to read and write them. `ParseMap` returns the
+  plain `map[string]any` tree, the shape `Parse` used to give. `Marshal` does
+  not accept a `Document`; it writes values, so `doc.Map()` is the way through.
 - `OffsetDateTime`, the Go type of the offset date-time kind, so that all four
   TOML date-time kinds have one of their own. `Parse` and `Unmarshal` hand it
   back where they produced a bare `time.Time` before, and `Marshal` accepts it.