Update Help menu

Before	After
	Peek_2024-04-24_21-11 (ignore the grey-ing out, this is because I demoed it by just moving the ui file to my 1.4.x official install)

This MR:

• adds Beginners' guide
• adds Extension dev guide
• uses redirects for manual, extension dev guide and beginners' guide (already added to inkscape.org)
• restructures the Help menu in order of access frequency
• removes access to 'About Memory' dialog from Help menu
• could be modified to remove the dialog itself, too, but that needs dev's opinions.