Move documentation from SmartSim repository

[ankona]

Moving existing documentation out of SmartSim repository to be maintained within the system-of-record. This includes:

• adding a make target to build the documentation
• moving docs to the standard <repo>/docs folder structure
• copypasta of SmartSim .gitignore to ignore build outputs
• adding doc dependencies to pyproject.toml to install sphinx/build reqs
• adding basic index.rst and placeholder changelog.rst