Add Custom Client example to the docs

[harishmohanraj]

Fixes #424

Type of change

• Documentation (typos, code examples, or any documentation updates)

[codecov]

Codecov Report

Attention: Patch coverage is 62.50000% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ting_started/fastapi/main_custom_fastapi_client.py	62.50%	6 Missing ⚠️

Files with missing lines	Coverage Δ
...ting_started/fastapi/main_custom_fastapi_client.py	62.50% <62.50%> (ø)

... and 4 files with indirect coverage changes

[davorinrusevljan]

Client sending response to the server if required by received message

[davorinrusevljan]

Server processes sends the next workflow message

[davorinrusevljan]

you can also say that most other languages and frameworks could be using this, like go, java,..