docs/clarify SSR body and layout APIs

[thescientist13]

Related Issue

Got some feedback in Discord that some of the docs around SSR pages and layouts wasn't clear in certain areas. For example, in the case of SSR pages, it was not clear the relationship between default export and getBody APIs, which used to be a bit clearer on the old website

Summary of Changes

1. Clarify relationship between default export and getBody SSR page APIs
2. Realized there wasn't an example of a basic dynamic layout

TODO

1. Confirm support of custom resources for layouts (e.g. TypeScript, src/pages/app.ts) like we do for SSR pages and API routes - https://github.com/ProjectEvergreen/greenwood/tree/master/packages/plugin-typescript#pages - support custom resource plugin formats for layouts greenwood#1395
2. confirm / clarify expectations around frontmatter layouts mapping - dynamic runtime layouts for SSR greenwood#1248 (comment)

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