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get_sites() will get a list of the sites in a Multisite network. You can then use switch_to_blog()[*], get_posts() / get_pages(), and restore_current_blog()[*] to get the pages from the various sites.
I’d strongly recommend storing the results as a sitewide option using update_site_option() to lessen the load on your server. You could add a job to wp_cron() to update your option periodically, or add an action to the save_post (or one or more appropriate Post Status Transition hook(s), such as new_to_publish and draft_to_publish) to update your stored menu list. An added advantage to keeping the menu list in a site option is that you can access it from any site in your network using get_site_option().
[*] — When Multisite originally came out, the terminology was a site of blogs; later it changed to a network of sites, but the old ways still appear in function names like switch_to_blog() and restore_current_blog().
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