Add documentation for slash commands in docs/slash_commands.md. (#5685)

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.

docs/slash_commands.md

@@ -0,0 +1,31 @@
+## Slash Commands
+
+### What are slash commands?
+
+Slash commands are special commands you can type that start with `/`.
+
+---
+
+### Built-in slash commands
+
+Control Codex’s behavior during an interactive session with slash commands.
+
+| Command      | Purpose                                                     |
+| ------------ | ----------------------------------------------------------- |
+| `/model`     | choose what model and reasoning effort to use               |
+| `/approvals` | choose what Codex can do without approval                   |
+| `/review`    | review my current changes and find issues                   |
+| `/new`       | start a new chat during a conversation                      |
+| `/init`      | create an AGENTS.md file with instructions for Codex        |
+| `/compact`   | summarize conversation to prevent hitting the context limit |
+| `/undo`      | ask Codex to undo a turn                                    |
+| `/diff`      | show git diff (including untracked files)                   |
+| `/mention`   | mention a file                                              |
+| `/status`    | show current session configuration and token usage          |
+| `/mcp`       | list configured MCP tools                                   |
+| `/logout`    | log out of Codex                                            |
+| `/quit`      | exit Codex                                                  |
+| `/exit`      | exit Codex                                                  |
+| `/feedback`  | send logs to maintainers                                    |
+
+---