docs: update the titles for all files

docs/development/adaptive_storage_system.md

@@ -1,3 +1,5 @@
+# Writing your retrieval system
+
 Scrapling uses SQLite by default, but this tutorial shows how to write your own storage system to store element properties for the `adaptive` feature.
 
 You might want to use Firebase, for example, and share the database between multiple spiders on different machines. It's a great idea to use an online database like that because spiders can share adaptive data with each other.

docs/development/scrapling_custom_types.md

@@ -1,3 +1,5 @@
+# Using Scrapling's custom types
+
 > You can take advantage of the custom-made types for Scrapling and use them outside the library if you want. It's better than copying their code, after all :)
 
 ### All current types can be imported alone, like below