Please add README to CLI pages directory

[dandv]

I wanted to add a description to the deno add page, pointing to more/related more information.

When I clicked the "Edit this page" link at the bottom, I landed on an empty file, https://github.com/denoland/docs/blob/main/runtime/reference/cli/add.md

It should be documented somewhere (ideally in that file as well),

1. Why is that file empty when other CLI files are not?
2. How is it generated?
3. How can contributors add to it (should they?)

Then I went to the file's parent directory, https://github.com/denoland/docs/tree/main/runtime/reference/cli. It would help to have a README there, even if it just points to index.md.