docs: add a basic tutorial for using the catalog and clients depending on the downloader (Pooch and DownloadManager)

Author: ecarrenolozano

docs/learn/tutorials/tutorial0001_basics.md

@@ -0,0 +1,74 @@
+# Basics: Downloading Ontologies
+
+This minimal tutorial shows how to download ontologies using either Pooch or Download Manager, and how to load from the catalog or a URL.
+
+---
+
+## 1. Choose a Global Default Downloader
+
+Set the default once in `ontograph/config/settings.py`:
+
+```python
+DEFAULT_DOWNLOADER = "pooch"
+# or
+DEFAULT_DOWNLOADER = "download_manager"
+```
+
+Now any client will use the configured backend unless you pass a downloader explicitly.
+
+---
+
+## 2. Use Pooch Explicitly
+
+```python
+from ontograph.client import ClientCatalog, ClientOntology
+from ontograph.downloader import PoochDownloaderAdapter
+from ontograph.config.settings import DEFAULT_CACHE_DIR
+
+downloader = PoochDownloaderAdapter(cache_dir=DEFAULT_CACHE_DIR)
+
+catalog = ClientCatalog(cache_dir="./data/out", downloader=downloader)
+catalog.load_catalog()
+
+client = ClientOntology(cache_dir="./data/out", downloader=downloader)
+client.load(source="go")  # catalog download
+```
+
+---
+
+## 3. Use Download Manager Explicitly
+
+```python
+from ontograph.client import ClientCatalog, ClientOntology
+from ontograph.downloader import DownloadManagerAdapter
+from ontograph.config.settings import DEFAULT_CACHE_DIR
+
+downloader = DownloadManagerAdapter(
+    cache_dir=DEFAULT_CACHE_DIR,
+    backend="requests",
+)
+
+catalog = ClientCatalog(cache_dir="./data/out", downloader=downloader)
+catalog.load_catalog()
+
+client = ClientOntology(cache_dir="./data/out", downloader=downloader)
+client.load(source="go")  # catalog download
+```
+
+---
+
+## 4. Download From a URL
+
+```python
+from ontograph.client import ClientOntology
+from ontograph.downloader import PoochDownloaderAdapter
+from ontograph.config.settings import DEFAULT_CACHE_DIR
+
+downloader = PoochDownloaderAdapter(cache_dir=DEFAULT_CACHE_DIR)
+
+# URL to GO ontology
+source_go = "https://purl.obolibrary.org/obo/go.obo"
+
+client = ClientOntology(cache_dir=DEFAULT_CACHE_DIR, downloader=downloader)
+client.load(source=source_go)
+```