Add documentation for API header info and more on document API

Author: hoijnet

src/app/docs/document-graph-api/page.md

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+---
+title: Document Graph API Howto
+nextjs:
+  metadata:
+    title: Document Graph API Howto
+    description: A howto guide for the TerminusDB document API interface.
+    alternates:
+      canonical: https://terminusdb.org/docs/document-graph-api-howto/
+media: []
+---
+
+The TerminusDB document interface blends knowledge graphs with document databases and realize the best parts of both, and offers a database-oriented approach to knowledge graph processing, where the complex and painful handling of relational tables and the inflexibiltiy of working with complete documents in a document databases are avoided.
+
+Documents in TerminusDB follow a strict schema which makes document modelling both precise and easy. Technical professionals will immediately feel at home with a JSON syntax, extended with types and identifiers for precise handling of both top level document, and subdocument handling.
+
+## Documents offers the best of graph, documents and relations
+
+The document interface implements an opionionated subset of the JSON-LD standard, with automatic ID generation and schemaful handling of documents and subdocuments, to form a special-purpose database for closed world assumption knowledge graph processesing. 
+
+The document interface consists of two endpoints. The first endpoint, `document`, is how we get documents into and out of TerminusDB. Since schemas consist of documents too, this is also how you'd update the schema, they use the same semantics.
+
+The second endpoint, `schema`, is how we can easily get schema information out of TerminusDB. While technically it is possible to get all schema information through the document interface, the schema interface is more convenient for this purpose, as it takes class inheritance into account to give a complete image of all the properties that are usable on a certain class.
+
+## Using the document API
+
+The easiest way to get started with the document API is to use the TerminusDB client libraries, which provide a high-level interface to the document API:
+
+* [Javascript](/docs/use-the-javascript-client/)
+* [Python](/docs/use-the-python-client/)
+
+## Further Reading
+
+* [Document Insertion Reference Guide](/docs/document-insertion/)
+* [Documents in a knowledge graph and how to use them](/docs/documents-explanation/)
+* [Immutability, version history, and revision control](/docs/immutability-explanation/)