insert an a Pages newsletter I would like to insert into a Apple Mailing

You can choose File menu : Export To : PDF - Send a Copy : Mail and a new Mail compose window will appear with the exported newsletter PDF attached.

In contrast, File menu : Share… Send Copy : Mail will attach a Pages document, not something that everyone can read without access to a current Pages application on macOS, iOS, iPadOS, or iCloud.

If the Pages newsletter was simply text, you could copy/paste it into a Mail compose window. Any text with a link associated with it will paste into Apple mail with the blue link appearance, but inactive. Equations will paste as PDF attachments. Any other selected document objects such as images, text boxes, shapes, charts, tables, etc. will not copy and paste into Mail.

Even if you chose to export the one-page newsletter as an image, that image would be an attachment in Mail. Although a one-page PDF would appear open and readable in your Mail compose window, it will be treated as an attachment upon receipt by other users email clients — though I suggested PDF because everyone has the ability to read it in the original Pages formatting.