docs: Add instructions to generate sphinx reference docs to CONTRIBUTING guide

Description

Updated the contributing guide to include instructions on how to generate sphinx reference documentation locally

Fixes #(issue) - N/A

Why is the change necessary?

Sphinx reference docs can't be rendered directly in your IDE and it is useful to be able to generate the docs locally to visualize your changes before merging them. For example, we use autodoc to document classes - these instructions will make it possible to visualize the documentation locally (including docstrings)

Solution

Provide instructions to generate the documentation in /doc locally based on PyCharm's Generating Reference Documentation Using Sphinx

Note: We don't need to do Tools | Sphinx quickstart since we already have a conf.py file in the /doc directory

Also in this PR:

Fixed docstring links in sagemaker.py and service.py
Added checklist item to PR template to generate sphinx documentation
Testing

Generated the documentation locally following the added instructions

Pull Request Checklist

Please check all boxes (including N/A items)

Testing

[X] Unit tests added - N/A
[X] Integration test added - N/A
[X] Manual testing - why was it necessary? could it be automated? - N/A

Documentation

[X] docs: All relevant docs updated
[X] docstrings: All public APIs documented

Title and description

[X] Change type: Title is prefixed with change type: and follows conventional commits
[X] References: Indicate issues fixed via: Fixes #xxx - N/A

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license.

aws / aws-step-functions-data-science-sdk-python