Dutch National Flag Algorithm [CPP]

#### This issue is about [Dutch National Flag Algorithm](https://medium.com/@warriorak77/dutch-national-flag-algorithm-44e3160c3740).

- [x] Issue Title is in form `Algo/DS Name [Language]`.
- [x] I searched or browsed the repo’s other issues to ensure this is not a duplicate.
- [x] This Algo/DS is worth adding to this repository.
- [x] Assign this issue to me (I want to work on this).
- [x] I ensure that I am not already assigned to 2 or more issues.
- [x] I will send my PR only after I'm assigned to this issue by any maintainer.
- [x] I understand that I will be unassigned from this issue if I fail to send a PR for this within a week.

Read the linked Dutch National Flag Algorithm article and inspect existing C++ algorithm implementations in iiitv/algos. Confirm the repository’s expected location and validation steps before adding the algorithm; done means a reviewed C++ implementation matching those conventions.

Written by the indexing model from the issue text.