reorganize docs into sections for easier navigation part 1

docs/relations/many-to-many.md

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+# ManyToMany
+
+`ManyToMany(to, through)` has required parameters `to` and `through` that takes target and relation `Model` classes.  
+
+Sqlalchemy column and Type are automatically taken from target `Model`.
+
+* Sqlalchemy column: class of a target `Model` primary key column  
+* Type (used for pydantic): type of a target `Model` 
+
+## Defining Models
+
+```Python
+--8<-- "../docs_src/relations/docs002.py"
+```
+
+Create sample data:
+```Python
+guido = await Author.objects.create(first_name="Guido", last_name="Van Rossum")
+post = await Post.objects.create(title="Hello, M2M", author=guido)
+news = await Category.objects.create(name="News")
+```
+
+### add
+
+```python
+# Add a category to a post.
+await post.categories.add(news)
+# or from the other end:
+await news.posts.add(post)
+```
+
+!!!warning
+    In all not None cases the primary key value for related model **has to exist in database**.
+    
+    Otherwise an IntegrityError will be raised by your database driver library.
+
+### remove
+
+Removal of the related model one by one.
+
+Removes also the relation in the database.
+
+```python
+await news.posts.remove(post)
+```
+
+### clear
+
+Removal of all related models in one call.
+
+Removes also the relation in the database.
+
+```python
+await news.posts.clear()
+```
+
+### QuerysetProxy
+
+Reverse relation exposes QuerysetProxy API that allows you to query related model like you would issue a normal Query.
+
+To read which methods of QuerySet are available read below [querysetproxy][querysetproxy]
+
+
+[queries]: ./queries.md
+[querysetproxy]: ./queryset-proxy.md
+[get]: ./queries.md#get
+[all]: ./queries.md#all
+[create]: ./queries.md#create
+[get_or_create]: ./queries.md#get_or_create
+[update_or_create]: ./queries.md#update_or_create
+[filter]: ./queries.md#filter
+[exclude]: ./queries.md#exclude
+[select_related]: ./queries.md#select_related
+[prefetch_related]: ./queries.md#prefetch_related
+[limit]: ./queries.md#limit
+[offset]: ./queries.md#offset
+[count]: ./queries.md#count
+[exists]: ./queries.md#exists
+[fields]: ./queries.md#fields
+[exclude_fields]: ./queries.md#exclude_fields
+[order_by]: ./queries.md#order_by