🐛 Fix hardcoded docs URL

[YuriiMotov]

Closes: #162

FastAPI allows configuring the docs URLs, but fastapi-cli currently doesn't respect it:

from fastapi import FastAPI

app = FastAPI(docs_url="/swagger_docs", root_path="/api")

@app.get("/")
async def root():
    return {"message": "Hello World"}

(docs URL should be http://0.0.0.0:8000/api/swagger_docs)

$ fastapi run

   FastAPI   Starting production server 🚀

  ...
 
    server   Server started at http://0.0.0.0:8000
    server   Documentation at http://0.0.0.0:8000/docs

This PR fixes this.

Use cases:

• disable API docs by setting openapi_url=None
• disable Swagger docs by setting docs_url=None, Redoc docs URL will be shown
• Custom docs URLs for Swagger docs and for Redoc
• root_path specified as an option to fastapi run\dev command or as a parameter to FastAPI

With this fix:

$ fastapi run

   FastAPI   Starting production server 🚀
 
  ...
 
    server   Server started at http://0.0.0.0:8000/api
    server   Documentation at http://0.0.0.0:8000/api/swagger_docs

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See also #381 that solves the issue with docs being unreachable via links due to 0.0.0.0 in host address

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This PR is based on #119, credits to @FlavienRx