function performImport() {
  // Gets the folder in Drive associated with this application.
  const folder = getFolderByName_(PROJECT_FOLDER_NAME);
  // Gets the Google Docs files found in the folder. 
  const files = getFiles(folder);

  // Warns the user if the folder is empty.
  const ui = DocumentApp.getUi();
  if (files.length === 0) {
    const msg =
      `No files found in the folder '${PROJECT_FOLDER_NAME}'.
      Run '${MENU.SETUP}' | '${MENU.SAMPLES}' from the menu
      if you'd like to create samples files.`
    ui.alert(APP_TITLE, msg, ui.ButtonSet.OK);
    return;
  }

  /** Processes main document */
  // Gets the active document and body section.
  const docTarget = DocumentApp.getActiveDocument();
  const docTargetBody = docTarget.getBody();

  // Appends import summary section to the end of the target document. 
  // Adds a horizontal line and a header with today's date and a title string.
  docTargetBody.appendHorizontalRule();
  const dateString = Utilities.formatDate(new Date(), Session.getScriptTimeZone(), 'MMMM dd, yyyy');
  const headingText = `Imported: ${dateString}`;
  docTargetBody.appendParagraph(headingText).setHeading(APP_STYLE);
  // Appends a blank paragraph for spacing.
  docTargetBody.appendParagraph(" ");

  /** Process source documents */
  // Iterates through each source document in the folder.
  // Copies and pastes new updates to the main document.
  let noContentList = [];
  let numUpdates = 0;
  for (let id of files) {

    // Opens source document; get info and body.
    const docOpen = DocumentApp.openById(id);
    const docName = docOpen.getName();
    const docHtml = docOpen.getUrl();
    const docBody = docOpen.getBody();

    // Gets summary content from document and returns as object {content:content}
    const content = getContent(docBody);

    // Logs if document doesn't contain content to be imported.
    if (!content) {
      noContentList.push(docName);
      continue;
    }
    else {
      numUpdates++
      // Inserts content into the main document.
      // Appends a title/url reference link back to source document.
      docTargetBody.appendParagraph('').appendText(`${docName}`).setLinkUrl(docHtml);
      // Appends a single-cell table and pastes the content.
      docTargetBody.appendTable(content);
    }
    docOpen.saveAndClose()
  }
  /** Provides an import summary */
  docTarget.saveAndClose();
  let msg = `Number of documents updated: ${numUpdates}`
  if (noContentList.length != 0) {
    msg += `\n\nThe following documents had no updates:`
    for (let file of noContentList) {
      msg += `\n ${file}`;
    }
  }
  ui.alert(APP_TITLE, msg, ui.ButtonSet.OK);
}
Updates the main document, importing content from the source files.
Uses the above parameters to locate content to be imported.

Called from menu option.