Docs: improve API Reference -> File System Conventions index page

[kcrwfrd]

Goals

1. Make it easier to quickly reference file system conventions at a glance, in API reference format

Background

I always find myself on https://nextjs.org/docs/app/api-reference/file-conventions when the page I really wanted was https://nextjs.org/docs/app/getting-started/project-structure

The project structure page is a million times better as an exhaustive file-system conventions index page, but I always forget where to find it because it's under "guides" rather than the more aptly named "API Reference -> File System Conventions"

Proposal

Either the pages should be reorganized or the file conventions page should be in an improved "cheat-sheet" index format similar to the project structure guide.