baseUri 
  Uri that relative paths are resolved relative to.
If the baseUri ends with /, it is considered a directory and relative paths in the markdown are resolved relative to that directory:
const md = new vscode.MarkdownString(`[link](./file.js)`);
md.baseUri = vscode.Uri.file('/path/to/dir/');
// Here 'link' in the rendered markdown resolves to '/path/to/dir/file.js'Content copied to clipboard
If the baseUri is a file, relative paths in the markdown are resolved relative to the parent dir of that file:
const md = new vscode.MarkdownString(`[link](./file.js)`);
md.baseUri = vscode.Uri.file('/path/to/otherFile.js');
// Here 'link' in the rendered markdown resolves to '/path/to/file.js'Content copied to clipboard