text: Title: Add convenience `get_sentence_vector()`-like methods for FastText, other models
  Body: Per <https://stackoverflow.com/questions/65397810/whats-the-equivalent-to-get-sentence-vector-for-gensims-fasttext>, the official Python FastText wrapper offers a `get_sentence_vector()` convenience method to averaging the word-vectors of a text. 
  
  Gensim could offer something similar, for FastText & other models. Though, it should perhaps have a more generic name & clear docs that this is just one simple way to create a text-vector from a bunch of words. 