Add documentation for slash commands in docs/slash_commands.md.

[pygarap]

This pull request adds a new documentation section to explain the available slash commands in Codex. The update introduces a clear overview and a reference table for built-in commands, making it easier for users to understand and utilize these features.

Documentation updates:

• Added a new section to docs/slash_commands.md describing what slash commands are and listing all built-in commands with their purposes in a formatted table.

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[pygarap]

I have read the CLA Document and I hereby sign the CLA

[etraut-openai]

Thanks for the contribution.

I think this is a good addition to the docs, but this new page isn't linked from anywhere. Could you add a link from the main documentation table of contents in README.md? Thanks!

[pygarap]

@etraut-openai Thanks for the review and the helpful note. I’ve added a link from the table of contents in README.md to the slash_commands.md page, as you suggested. I appreciate you taking the time to look into it.

While I was there, I also added documentation for the new /undo and /exit slash commands, which were not included in the version you reviewed. Please let me know if you’d like any changes.

[etraut-openai]

@pygarap, there's one CI failure (a formatting issue) that needs to be fixed. Otherwise it looks good to me.

[pygarap]

@etraut-openai Thank you for noting the CI issue. I have run pnpm run format --write docs/slash_commands.md, and the formatting issue is now resolved. This should clear the CI formatting check. Please let me know if you’d like any further changes.