Fix/restore `prompting_guide.md`, or remove link to it in `README.md`

What is the type of issue?

Documentation is missing

What is the issue?

There used to be a prompting_guide.md:

• https://github.com/openai/codex/commits/main/codex-cli/examples/prompting_guide.md

But that was seemingly lost when the typescript code was removed:

• https://github.com/openai/codex/pull/2048

Yet it is still referenced/linked to in places such as the main README.md, which is confusing for new users:

https://github.com/openai/codex/blob/1930ee720ac4c8873d68c1b046de055755369332/README.md?plain=1#L229-L231

I didn't search comprehensively to see if it was reference from other locations.

Where did you find it?

https://github.com/openai/codex/blob/1930ee720ac4c8873d68c1b046de055755369332/README.md?plain=1#L229-L231