docs: document SHELL_VERBOSITY environment variable

Add documentation explaining how to set verbosity level using the
SHELL_VERBOSITY environment variable. This is useful in CI/CD pipelines
or scripts where command-line arguments cannot be easily modified.

The note includes:
- Table showing verbosity levels and their equivalents
- Example usage for setting debug output

Closes #7601

---
Ahmed Adel Bakr Alderai

Author: Ahmed-AdelB

docs/cli.md

@@ -21,6 +21,25 @@ then `--help` combined with any of those can give you more information.
 ## Global Options
 
 * `--verbose (-v|vv|vvv)`: Increase the verbosity of messages: "-v" for normal output, "-vv" for more verbose output and "-vvv" for debug.
+
+{{% note %}}
+You can also set the verbosity level using the `SHELL_VERBOSITY` environment variable.
+This is useful in CI/CD pipelines or scripts where you cannot easily modify command-line arguments.
+
+| Value | Equivalent | Description |
+|-------|------------|-------------|
+| `-1` | `-q` | Quiet mode |
+| `0` | (default) | Normal output |
+| `1` | `-v` | Verbose output |
+| `2` | `-vv` | More verbose output |
+| `3` | `-vvv` | Debug output |
+
+For example, to enable debug output:
+```bash
+export SHELL_VERBOSITY=3
+poetry install
+```
+{{% /note %}}
 * `--help (-h)` : Display help information.
 * `--quiet (-q)` : Do not output any message.
 * `--ansi`: Force ANSI output.