Update adjusting_sample_to_population.md with current Sample.adjust() output format (#315)

Summary:
The documentation showed outdated output format for `Sample.adjust()`. Updated to reflect current behavior from the quickstart tutorial.

## Changes

- **Added adjustment details section** with design effect (Deff), effective sample size proportion (ESSP), effective sample size (ESS), method, and weight trimming mean ratio
- **Updated target metadata** to correctly show 3 variables (gender, age_group, income) with `outcome_columns: happiness`, matching the quickstart tutorial where target is created with `outcome_columns=["happiness"]`
- **Updated formatting** to match current string representation

## Example

Current output now shows:
```
        Adjusted balance Sample object with target set using ipw
        1000 observations x 3 variables: gender,age_group,income
        id_column: id, weight_column: weight,
        outcome_columns: happiness

        adjustment details:
            method: ipw
            weight trimming mean ratio: 20
            design effect (Deff): 1.880
            effective sample size proportion (ESSP): 0.532
            effective sample size (ESS): 531.9

            target:

	        balance Sample object
	        10000 observations x 3 variables: gender,age_group,income
	        id_column: id, weight_column: weight,
	        outcome_columns: happiness

            3 common variables: gender,age_group,income
```

<details>

<summary>Original prompt</summary>

>
> ----
>
> *This section details on the original issue you should resolve*
>
> <issue_title>[FEATURE] website/docs/docs/general_framework/adjusting_sample_to_population.md</issue_title>
> <issue_description>Update text in:
> https://github.com/facebookresearch/balance/blob/main/website/docs/docs/general_framework/adjusting_sample_to_population.md
> Based on updated output from here:
> https://import-balance.org/docs/tutorials/quickstart/</issue_description>
>
> ## Comments on the Issue (you are copilot in this section)
>
> <comments>
> </comments>
>

</details>

- Fixes #314

 ---

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Pull Request resolved: #315

Reviewed By: omriharosh

Differential Revision: D92278189

Pulled By: talgalili

fbshipit-source-id: e2e5acc095041e2585c48b3e5f57527002ff6215

Author: Copilot

website/docs/docs/general_framework/adjusting_sample_to_population.md

@@ -15,19 +15,26 @@ adjusted = sample.adjust()
 The output of this method is an adjusted `Sample` class object of the form:
 
 ```
-    Adjusted balance Sample object with target set using ipw
-    1000 observations x 3 variables: gender,age_group,income
-    id_column: id, weight_column: weight,
-    outcome_columns: happiness
+        Adjusted balance Sample object with target set using ipw
+        1000 observations x 3 variables: gender,age_group,income
+        id_column: id, weight_column: weight,
+        outcome_columns: happiness
 
-        target:
+        adjustment details:
+            method: ipw
+            weight trimming mean ratio: 20
+            design effect (Deff): 1.880
+            effective sample size proportion (ESSP): 0.532
+            effective sample size (ESS): 531.9
 
-            balance Sample object
-            10000 observations x 3 variables: gender,age_group,income
-            id_column: id, weight_column: weight,
-            outcome_columns: None
+            target:
 
-        3 common variables: income,gender,age_group
+                balance Sample object
+                10000 observations x 3 variables: gender,age_group,income
+                id_column: id, weight_column: weight,
+                outcome_columns: happiness
+
+            3 common variables: gender,age_group,income
 ```
 Note that the `adjust` method in balance is performing three main steps:
 1. **Pre-processing** of the data - getting data ready for adjustment using best practices in the field: