Stable document format MVP

Stable document format:
- Infrastructure for migrating old to new formats
- Separate structure for storing the document, decoupled from the `struct DocumentMessageHandler` and `struct NodeNetwork` definitions
- One struct per version, plus an associated upgrade function that goes to the next version
- ASCII graph structure markup format (use RON syntax, we need to define the semantics), see #1605
- Translation back from the ASCII format to those structs
- A way to look up a built-in node's definition by its name and not have to store its sub-network
- A way to store user-created subgraph network (or modified from a built-in node definition)

Start by reading issue #1605 and the existing DocumentMessageHandler and NodeNetwork definitions mentioned here. Determine the format semantics, versioning and upgrade boundaries, then scope how built-in and user-created subgraphs are represented. Done requires agreement on the design and implementation of the listed migration, serialization, lookup and translation capabilities.

Vom Indexierungsmodell aus dem Issue-Text verfasst.