add a README-file for the doc subdirectory

While migrating the APIv3 documentation from https://github.com/opf/op-api-v3-documentation, the readme-file was lost.

This PR adds a README for the `doc` subdirectory, which primarily contains the contents of the old APIv3 README.

GitHub should be able to render this readme-file as soon as you [enter the doc-subdirectory](https://github.com/opf/openproject/tree/8d535a4e124e7ceec2ec3e9fd3fefe7204fe620a/doc).