Add markdown formatting to CI

When users open a pull request, they have to include a file `README.body.md` that contains a detailed description of the model. (For details, see the [section in the contributing guide](https://github.com/KhronosGroup/glTF-Sample-Assets/blob/main/CONTRIBUTING.md#the-readme-body))

There are some requirements for "proper" formatting of these files. For the _existing_ `body.md` files, this is fixed via https://github.com/KhronosGroup/glTF-Sample-Assets/pull/246 . We should consider adding a CI step that integrates some standard markdown linting/formatting tool and that automatically formats these `body.md` files accordingly.

Read the README.body.md requirements in CONTRIBUTING.md and review the existing formatting change in pull request 246. Then identify the repository's CI configuration and determine how a standard Markdown linting or formatting tool should process existing body.md files; done means the CI step automatically formats them according to the documented requirements.

Written by the indexing model from the issue text.