DocLang backlog

- [x] **❗ High prio ❗**
- [ ] training-relevant
- [ ] decide on heading token (XML prolog, version)
- [ ] align w.r.t. tokenizer
- [ ] update draft
- [ ] make simpler (no redundant listings!) & stricter
- [ ] align to recent updates (`content`, `base64`, layer classes, picture classes etc)
- [ ] decide on ambiguous parts (e.g. `group`, `br`)
- [ ] update schema (XSD + Schematron)
- [ ] extend schematron rules to address plain text in OTSL and others

- [x] **🔸 Medium prio 🔸**
- [ ] improve kv e2e in case of picture with text as [key](https://github.com/docling-project/docling-core/blob/main/test/data/doc/kv/08212053e2db1a70dd60a4f85650ceb33d7519af34f502e3ac894389d76663d6/output.dclg.xml#L398-L424) or [value](https://github.com/docling-project/docling-core/blob/main/test/data/doc/kv/08212053e2db1a70dd60a4f85650ceb33d7519af34f502e3ac894389d76663d6/output.dclg.xml#L99-L125)
- [ ] address cross-references
- [ ] consider serialization extensions
- [ ] alternative export mode where KVs are shown as just text
- [ ] consider standard extensions
- [ ] support extended bounding polygons (beyond non-rotated rectangles) (see #348)
- (on draft level, could just be by generalizing from 4=2*2 location tokens (interpreted as rectangle) to N*2 tokens for any arbitrary polygon with N vertices)
- [ ] update deserialization

- [x] **🔹 Low prio 🔹**
- [ ] DocLang (embedded) archive
- [ ] page breaks
- [ ] comments / refs to partial spans
- [ ] check formatting:hyperlink (#578)
- [ ] list languages in draft (as subset of Linguist vX.Y.Z)
- [ ] extend list to support e.g. Markdown?
- [ ] define clearly how non-programming log/console-like content is to be captured (e.g. `code` with unset language or `Shell`?)
- [ ] cross-provenance content
- [ ] cross-page content
- [ ] misc
- [ ] clarify dropping of `include_formatting` serializer switch
- [x] **✅ Done ✅**
- [x] address key value regions (consolidate `FormItem` / `KeyValueItem` (`KEY_VALUE_REGION`) (#519)
- [x] fix table cell content deserialization (#512)
- [x] fix document re-indexing (#510)
- [x] fix DocTags picture meta deserialization (#505)
- [x] review checkboxes (#503)
- [x] review chart serialization (compared to table serialization) (#498)
- [x] align handling of default resolution (location) (#492)
- [x] rich tables / nested tables (#491)
- [x] leading / trailing whitespaces (#489)
- [x] finalize outmost element name (#494)
- [x] ~~add radio button support~~ (update: not for now)
- [x] XML Schema / DTD?
- [x] converge between `` and {``+ other way of signalizing whitespace preservation e.g. via `ATTLIST`, XML schema, convention etc.}
- [x] line breaks (#575)
- [x] content layer (e.g. furniture) not captured in DocLang (#568)
- [x] map HANDWRITTEN label to new DocLang element (similar to bold, italics) (#561)
- [x] inline code issue: conflict between `` and `` (#517)
- [x] use defusedxml? (#509)
- [x] align image ref mode on DocLang serialization (#506)
- [x] image URI serialization (#504)
- [x] track CVAT test output (docling-project/docling-cvat-tools/pull/8)