(wip) docs: revamp conceptual docs (#1663)

* docs: revamp conceptual docs

* multi-agent draft

* typo

* Update Why LangGraph page

* update multi-agent

* Update persistence page

* update

* Clarification of graph re-playing

* update persistence

* more updates

* more updates

* Add multiple schemas section to glossary

* more updates

* Clarify I/O schema

* dynamic breakpoints

* Update agentic concepts

* Fix comments

* Update figureds

* cr

* cr

* cr

---------

Co-authored-by: Lance Martin <lance@langchain.dev>
Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

docs/docs/reference/checkpoints.md

@@ -1,13 +1,15 @@
-# Checkpoints
+# Checkpointers
 
-You can [compile][langgraph.graph.MessageGraph.compile] any LangGraph workflow with a [CheckPointer][basecheckpointsaver] to give your agent "memory" by persisting its state. This permits things like:
+You can [compile][langgraph.graph.MessageGraph.compile] any LangGraph workflow with a [Checkpointer][basecheckpointsaver] to give your agent "memory" by persisting its state. This permits things like:
 
 - Remembering things across multiple interactions
 - Interrupting to wait for user input
 - Resilience for long-running, error-prone agents
 - Time travel retry and branch from a previous checkpoint
 
-Key checkpointer interfaces and primitives are defined in [`langgraph_checkpoint`](https://github.com/langchain-ai/langgraph/tree/main/libs/checkpoint) library.
+Key checkpointer interfaces and primitives are defined in [`langgraph_checkpoint`](https://github.com/langchain-ai/langgraph/tree/main/libs/checkpoint) library. Additional checkpointer implementations are also available as installable libraries:
+* [`langgraph-checkpoint-sqlite`](https://github.com/langchain-ai/langgraph/tree/main/libs/checkpoint-sqlite): An implementation of LangGraph checkpointer that uses SQLite database. Ideal for experimentation and local workflows.  
+* [`langgraph-checkpoint-postgres`](https://github.com/langchain-ai/langgraph/tree/main/libs/checkpoint-postgres): An advanced checkpointer that uses Postgres database, used in LangGraph Cloud. Ideal for using in production.  
 
 ### Checkpoint
 
@@ -21,11 +23,17 @@ Key checkpointer interfaces and primitives are defined in [`langgraph_checkpoint
 
 ::: langgraph.checkpoint.base.BaseCheckpointSaver
 
+## Serialization / deserialization
+
 ### SerializerProtocol
 
 ::: langgraph.checkpoint.base.SerializerProtocol
 
-## Implementations
+### JsonPlusSerializer
+
+::: langgraph.checkpoint.serde.jsonplus.JsonPlusSerializer
+
+## Checkpointer Implementations
 
 LangGraph also natively provides the following checkpoint implementations.