Use variables for curl example base url with localhost:8000 as default

[woonsan]

I've added a configuration for curl example base url and let each rst source refer to the variable for interpolation.
As the variable is used in code block, I also had to replace .. code:: bash with .. parsed-literal::. curl command examples actually do not get much benefit from .. code:: bash, so I think .. parsed-literal:: is good enough for the purpose.
And, I removed the sentence mentioning script/test.sh and design/data_model page reference because they don't exist and make logs errors.
Please review the changes and merge it if okay.
Thanks!