Code format with the right subfolders

[xiaoyongzhu]

Goal of this PR is to add the right folder hierarchy for Feathr code (so that it can be better organized).

Other small improvements include:

• Updated the pydoc structure to make sure all the pydocs are in the right module name.
• Renaming frameconfig to feathrconfig and put it into the right folder structure
• Docs on how to expose the right module in pydocs

pydocs in this PR: https://feathr-xiaoyzhu.readthedocs.io/en/latest/index.html
pydocs in main branch: https://feathr.readthedocs.io/

We should also move the api folder out from the python client. Waiting for @blrchen and @YihuiGuo to fix those as there seems to be some dependencies. It's described at #287.

This should have no end user impact as end users are using this:
from feathr import X

The new file structure is like this:

(base) xiazhu@xiazhu-mn1 feathr % tree feathr_project/feathr -I __pycache__
feathr_project/feathr
├── __init__.py
├── api
│   ├── app
│   │   ├── __init__.py
│   │   ├── backends
│   │   │   ├── backend_base.py
│   │   │   └── purview_backend
│   │   │       ├── __init__.py
│   │   │       ├── purview_backend.py
│   │   │       └── purview_backend_config.py
│   │   ├── core
│   │   │   ├── __init__.py
│   │   │   ├── configs.py
│   │   │   ├── error_code.py
│   │   │   ├── error_handling.py
│   │   │   ├── feathr_api_exception.py
│   │   │   └── registry.py
│   │   ├── main.py
│   │   ├── models
│   │   │   └── features.py
│   │   └── routes
│   │       ├── feature.py
│   │       ├── job.py
│   │       ├── offline_feature.py
│   │       ├── online_feature.py
│   │       ├── project.py
│   │       └── transformation.py
│   └── requirements.txt
├── client.py
├── constants.py
├── definition
│   ├── _materialization_utils.py
│   ├── aggregation.py
│   ├── anchor.py
│   ├── dtype.py
│   ├── feathrconfig.py
│   ├── feature.py
│   ├── feature_derivations.py
│   ├── lookup_feature.py
│   ├── materialization_settings.py
│   ├── query_feature_list.py
│   ├── repo_definitions.py
│   ├── settings.py
│   ├── sink.py
│   ├── source.py
│   ├── transformation.py
│   └── typed_key.py
├── exception
├── protobuf
│   ├── __init__.py
│   └── featureValue_pb2.py
├── registry
│   ├── _feature_registry_purview.py
│   └── feature_registry.py
├── secrets
│   └── akv_client.py
├── spark_provider
│   ├── _abc.py
│   ├── _databricks_submission.py
│   ├── _synapse_submission.py
│   └── feathr_configurations.py
├── udf
│   ├── _preprocessing_pyudf_manager.py
│   └── feathr_pyspark_driver_template.py
└── utils
    ├── _envvariableutil.py
    ├── _file_utils.py
    ├── feature_printer.py
    └── job_utils.py

[hangfei]

Add a screenshot for the new structure?

[hangfei]

Could you provide a screenshot of the new py docs?

[hangfei]

Please test notebook as well since this changes a lot of structural stuff.

[xiaoyongzhu]

Please test notebook as well since this changes a lot of structural stuff.

Good catch. I've verified this branch with the latest notebook and all those worked.