feat(backend): refactor/cleaned/ infer get_serializer

* Cleaned all init files
* Infer the serializer from the model instead of having it in the models
* Updated the doc accordingly
* Fixed typos

Fixes #93
Fixes #85
6 jobs for !80 with enhance/backSimplicity in 4 minutes and 55 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Check
passed #481930
docker
check_back

00:00:21

passed #481931
docker
check_front

00:01:36

 
  Test
passed #481932
docker
test_back

00:00:44

88.0%
passed #481933
docker
test_frontend

00:00:58

 
  Lint
passed #481935
docker
eslint

00:01:01

passed #481934
docker
flake8

00:00:13