Enhance/custom_exports-api

[nifedara]

What does this PR do?

Enhances documentation quality

Makes updates to Custom Exports endpoints

• I added 200, 401, 403, 429, and 500 responses and examples
• I removed the trailing slash from the endpoint
• Contributes to resolving this issue: Enhance API documentation and examples hotosm/raw-data-api#219

Consideration :

• I was proactive in deducing and creating the 200-response example from the code
• I used black formatter to format the code

How to test?

• Clone this repo
• Follow Docker Installation Guide or local installation guide
• Shoot the server up and view the Documentation changes on your localhost