Update manifest concepts page

[zmitchell]

• Remove the list of manifest sections
• Add a link to the manifest reference page
• Add a short code block containing a manifest to be more in line with the other files listed on the same page

version  = 1

[install]
hello.pkg-path = "hello"

[options]
systems = ["aarch64-darwin", "x86_64-darwin", "aarch64-linux", "x86_64-linux"]