Can Marketo create a page that has subpages attached?

For me this comes down to answering the question:
Do all of these "pages" share the same URL?

IF YES:

You'll want to build a "single page" with content in tabs that can be shown/hidden based on clicks in a navigation menu or tab menu. The idea here is that a click on the nav/tab menu would show one "panel" at a time and you could swap those in out based on which link was clicked. 

PROS: Everything is contained on one page -- so it's usually a bit less to setup in terms of effort and maybe even a little more user-friendly b/c everything is in one place to just click around the page.

CONS: Everything is contained on one page. This can make it more difficult to setup your tracking and also more difficult to send a user to a specific panel on the page from an external link. Say you wanted to promote the "Rules" for some reason, it'd be harder to point the user to the "single page" and open the Rules tab than it would be to send them to a separate Rules page.

IF NO:
This means that there are actually several pages put together into what I've normally heard called a "microsite". In this case, you'd have several individual pages linked together with a navigation menu and when you clicked a link in the nav menu, it'd take you to the Rules "page" (rather than panel) for example. 
I think the easiest way to go about this in Marketo would be to use a Snippet for the header and footer so that those could be the same from page-to-page and then change what's in the body of the page to suit the use case - such as the Rules or Prizes or whatever. Using a snippet for the navigation menu will make it easier to maintain b/c you can manage the updates in one place rather than having to make changes to each page every time you make a change to the navigation menu. Basically, snippets are "one-to-many" rather than "one-to-one" in terms of the code-to-page.

PROS: Separate pages and snippets make it a bit easier to add things into the page as you go (more pages, more nav items) or remove them when not needed (just use less pages and hide the nav items). This setup is also a bit easier to track on the back end and makes it easier to link to specific parts of the information (like the Rules) compared to the single-page setup.

CONS: There's more pages to setup which can be a bit more effort, depending on how you look at it. For someone with a decent amount of HTML/CSS knowledge it might be easier to set it all up in a single-page app and manage it there. For someone who's less code-savvy it might be easier to manage it a page-at-a-time.

Sidenote: If you do end up using the multi-page approach, you'll want to make sure that your nav links open in the CURRENT WINDOW rather than in a NEW WINDOW. This'll make it act more like a website navigation menu and replace the current page with the link you clicked. This can be setup in the link settings and I'm pretty sure "NEW WINDOW" (target="_blank") is the default choice so it's something you'll probably have to change on each of the links.

First: “subpages” are not a standard HTML feature. Concepts like that have been implemented in various CMSes and web frameworks. But there’s no one meaning of the term.

I assume you mean (on a technical level) Can you replace blocks of content on a Marketo page with new content that wasn’t in the original document, without reloading the page?

If so, then yes. Although you have to write the code to do so, it’s not particularly hard. You can use additional Marketo LPs as content sources (they need only have a standard <div> container with a predictable id) and then load them via XHR, replacing current content. Those content source LPs are never accessed directly.

Alternately, you could add a mini-framework (I’ve used SquirrellyJS) to Marketo pages to help out, though it’s still not code-free.

In other words, you can build a single-page web app using Marketo LPs.