How to insert a separate page(s) for all subsites

Hello,
I want to create a separate html page which contains 2 levels (the page itself and some child-pages), and insert this page as a Page for all subsites. Could you please tell me
1. How to insert such page (located on the same server as WP) as a Page for all subsites? The page looks like a Page created by each subsite owner. Visiting it, users will be still in a subsite.
2. The html page will be updated frequently by me, and any change on it should be reflected in all subsites.

Thanks for your help!
BR
Y