Clean up file structure, devcontainer

[thekaveman]

• Move all site content / assets into a src/ directory
• Update the Jekyll config to build from this directory
• Update the devcontainer build to account for new directory if needed
• Update the devcontainer base image to python 3.11
• Make devcontainer use a non-root user
• .prettierrc to control indentation and other formatting

[machikoyasuda]

• use pyproject.toml (https://github.com/cal-itp/benefits/blob/dev/pyproject.toml)