Creating Topic Taxonomies

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### Feature request

Hey @MaartenGr,

first off: great repository! It was really easy to get into it and we're using BERTopic extensively, which brings me to my feature request (that I will follow up with a PR -> Update: [It's here :)](https://github.com/MaartenGr/BERTopic/pull/2527):

`create_topic_taxonomy()`: controllable multi-level topic hierarchies

We've been building topic taxonomies for our use case and ran into a gap between the existing options:

- hierarchical_topics() gives a full dendrogram but no control over the number of levels
- reduce_topics() can reduce to a target count but isn't really designed for building hierarchies - calling it repeatedly works but produces a lot of single-child branches and the quality of the hiearchy is questionable.

What we needed was a way to say *"give me 3 levels with roughly X, Y, Z topics each, and make sure every parent has at least N children."*

### Motivation

We're using BERTopic a lot, also in production, to analyze textual data, but we need levels and a topic hierarchy, which is why I'd like to contribute directly to the repo.

### Your contribution

So we built create_topic_taxonomy(), which:

- Takes nr_topics_per_level (e.g. [20, 5]) to define the shape
- Enforces a min_children constraint per parent (default 2) to avoid degenerate branches
- Optionally blends document centroids with topic embeddings (configurable weight) for richer topic vectors when building the hierarchy, which works really well for us.
- Clusters bottom-up using agglomerative clustering + a repair step for undersized clusters
- Labels parents via c-TF-IDF on merged documents
- Returns a clean DataFrame with Topic_ID, Level, Parent_ID, etc.
- Does not touch the fitted model - it's a read-only lookup table on top of the leaves

The method sits right next to hierarchical_topics() and follows existing code patterns. PR incoming with tests and docs.

---

We've added a dedicated documentation page to the mkdocs (Topic Taxonomy), the actual implementation is only a single method. :)

Start with the existing hierarchical_topics() implementation and PR #2527, which contains the proposed method. Review its accompanying tests and the mkdocs Topic Taxonomy page; done means the requested multi-level taxonomy behavior is covered and documented without modifying the fitted model.

Written by the indexing model from the issue text.