Update "page" template to elegantly display right nav

[jhaaaa]

Currently, any standalone pages on .org aren't able to elegantly display the "On this page" right navigation menu. For this reason, we hide the right nav, even if it would be easy and helpful to display it. (Standalone pages are those defined here: https://github.com/xmtp/xmtp-dot-org/tree/main/src/pages. These are not documentation pages nor blog posts.)

Here is what a standalone page looks like with the right nav hidden:

Here is what a standalone page looks like with the right nav displayed:

We would like the standalone page to display the main content area and right nav the same way as documentation pages. For example: