*   Since MkDocs 1.6 a file may alternatively be stored in memory - `content_string`/`content_bytes`.

    Then `src_dir` and `abs_src_path` will remain `None`. `content_bytes`/`content_string` need
    to be written to, or populated through the `content` argument in the constructor.

    But `src_uri` is still populated for such files as well! The virtual file pretends as if it
    originated from that path in the `docs` directory, and other values are derived.

For static files the file is just copied to the destination, and `dest_uri` equals `src_uri`.

For Markdown files (determined by the file extension in `src_uri`) the destination content
will be the rendered content, and `dest_uri` will have the `.html` extension and some
additional transformations to the path, based on `use_directory_urls`.
rš