docs: add exmaple of pivot_table and modify some contents

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Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
:white_check_mark: +0.00% (target: -5.00%)	:white_check_mark: ∅

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (f048640f6f6ac7864f7f5dd03d62081236fd0415) | 2879 | 2799 | 97.22% | | | Head commit (094420081077b8e72fa5273bc4f520882597faa3) | 2879 (+0) | 2799 (+0) | 97.22% (**+0.00%**) | **Coverage variation** is the difference between the coverage for the head and common ancestor commits of the pull request branch: ` - `

Diff coverage details

| | Coverable lines | Covered lines | Diff coverage | | ------------- | ------------- | ------------- | ------------- | | Pull request (#137) | 0 | 0 | **∅ (not applicable)** | **Diff coverage** is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: `/ * 100%`

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.97%. Comparing base (f048640) to head (0944200).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #137 +/- ## ======================================= Coverage 96.97% 96.97% ======================================= Files 26 26 Lines 2872 2872 ======================================= Hits 2785 2785 Misses 64 64 Partials 23 23 ```

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