docs: create guide to choose Embedders

[anakin87]

We have several Embedders on Haystack 2.0

• API based: OpenAI, Cohere, Jina...
• Sentence Transformers
• HF Text Embedding Inference
• Fastembed
• HF Optimum

Some focus on simplicity, others on supporting a wide range of models, and still others on performance on different architectures.

I think it would benefit users to have a guide, as we did for Generators:
this is not something that guides to choosing the best model,
but an acceptable Embedder based on one's needs/resources.