documentation: Pi Agent example has thinking modes wrong

The README says:

> DeepSeek V4 Flash has distinct non-thinking, thinking, and Think Max modes.

The same README has this in the Pi Agent configuration example:
```json
"thinkingLevelMap": {
"off": null,
"minimal": "low",
"low": "low",
"medium": "medium",
"high": "high",
"xhigh": "xhigh"
},
```

Assuming the non-thinking, thinking, thinkmax is right, there are these problems in the Pi example:
- `"off": null` one means "off is not supported, do not present it to the user", effectively masking the non-thinking mode from the users
- minimal trough xhigh are actually all mapped to "Thinking" by the model
- max is masked from the users

I have not yet been able to test it against `ds4`, but this mapping is more in line with the declared thinking modes:
```json
"thinkingLevelMap": {
"off": "off",
"minimal": null,
"low": null,
"medium": null,
"high": "high",
"xhigh": null,
"max": "max"
},
```