Add different data providers and improve docs

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

stores/filesystem/documents.go

@@ -0,0 +1,54 @@
+package filesystem
+
+import (
+	"bytes"
+	"context"
+	"excalidraw-complete/core"
+	"fmt"
+	"log"
+	"os"
+	"path/filepath"
+
+	"github.com/oklog/ulid/v2"
+)
+
+type documentStore struct {
+	basePath string // Directory where documents are stored.
+}
+
+func NewDocumentStore(basePath string) core.DocumentStore {
+	if err := os.MkdirAll(basePath, 0755); err != nil {
+		log.Fatalf("failed to create base directory: %v", err)
+	}
+
+	return &documentStore{basePath: basePath}
+}
+
+func (s *documentStore) FindID(ctx context.Context, id string) (*core.Document, error) {
+	filePath := filepath.Join(s.basePath, id)
+
+	data, err := os.ReadFile(filePath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, fmt.Errorf("document with id %s not found", id)
+		}
+		return nil, err
+	}
+
+	document := core.Document{
+		Data: *bytes.NewBuffer(data),
+	}
+
+	return &document, nil
+}
+
+func (s *documentStore) Create(ctx context.Context, document *core.Document) (string, error) {
+	id := ulid.Make().String()
+	filePath := filepath.Join(s.basePath, id)
+
+	if err := os.WriteFile(filePath, document.Data.Bytes(), 0644); err != nil {
+		return "", err
+	}
+
+	return id, nil
+}