divide docs in sections, provide Meta attributes inheritance, add tests for fastapi use wih mixins and concrete

docs/relations/index.md

@@ -45,11 +45,43 @@ class Department(ormar.Model):
 
 !!!tip
     Reverse ForeignKey allows you to query the related models with [queryset-proxy][queryset-proxy].
+    
+    It allows you to use `await department.courses.all()` to fetch data related only to specific department etc. 
 
 ##ManyToMany
 
 To define many-to-many relation use `ManyToMany` field.
 
+```python hl_lines="25-26"
+class Category(ormar.Model):
+    class Meta:
+        tablename = "categories"
+        database = database
+        metadata = metadata
+
+    id: int = ormar.Integer(primary_key=True)
+    name: str = ormar.String(max_length=40)
+
+# note: you need to specify through model
+class PostCategory(ormar.Model):
+    class Meta:
+        tablename = "posts_categories"
+        database = database
+        metadata = metadata
+
+class Post(ormar.Model):
+    class Meta:
+        tablename = "posts"
+        database = database
+        metadata = metadata
+
+    id: int = ormar.Integer(primary_key=True)
+    title: str = ormar.String(max_length=200)
+    categories: Optional[Union[Category, List[Category]]] = ormar.ManyToMany(
+        Category, through=PostCategory
+    )
+```
+
 
 !!!tip
     To read more about many-to-many relations visit [many-to-many][many-to-many] section
@@ -58,6 +90,8 @@ To define many-to-many relation use `ManyToMany` field.
 !!!tip
     ManyToMany allows you to query the related models with [queryset-proxy][queryset-proxy].
 
+    It allows you to use `await post.categories.all()` but also `await category.posts.all()` to fetch data related only to specific post, category etc.
+
 
 [foreign-keys]: ./foreign-key.md
 [many-to-many]: ./many-to-many.md