feat: update `handle_animations` docs

**Is your feature request relasted to a problem? Please describe.**
[this](https://github.com/imaNNeo/fl_chart/blob/master/repo_files/documentations/handle_animations.md) is deprecated

**Describe the solution you'd like**
Update md file

Start with repo_files/documentations/handle_animations.md and compare its guidance with the current animation API in fl_chart. Confirm which documented material is deprecated, then update the Markdown file so it describes the current approach and verify that no obsolete references remain.

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