Add different data providers and improve docs

This adds support for switching between in-memory, local filesystem, s3
or sqlite.

stores/memory/documents.go

@@ -0,0 +1,31 @@
+package memory
+
+import (
+	"context"
+	"excalidraw-complete/core"
+	"fmt"
+
+	"github.com/oklog/ulid/v2"
+)
+
+var savedDocuments = make(map[string]core.Document)
+
+type documentStore struct {
+}
+
+func NewDocumentStore() core.DocumentStore {
+	return &documentStore{}
+}
+
+func (s *documentStore) FindID(ctx context.Context, id string) (*core.Document, error) {
+	if val, ok := savedDocuments[id]; ok {
+		return &val, nil
+	}
+	return nil, fmt.Errorf("document with id %s not found", id)
+}
+
+func (s *documentStore) Create(ctx context.Context, document *core.Document) (string, error) {
+	id := ulid.Make().String()
+	savedDocuments[id] = *document
+	return id, nil
+}