no validation of sites.md and data.json in pull request workflow

[jguddas]

We should check:

1. When a PR alters the data.json that it is done so in accordance with the formatting provided by ./site_list.py.
2. When a PR adds or removes support for a site that the sites.md file is updated and formatted correctly.

Possible solution:

Adding a site_list.py step to the PR workflow with a new --fail-on-update option that exits with status code 1 when the formatting differs.