MMR support (#8690)

* initial clean commit

* updated docs and added unit test

* updated demo; bug fixes

* updated doc

* post running ruff

* updated documentation
added X-Source in headers.

* updated Vectara implementation:
- removed "Optional" from constructor and simplified how we deal with missing values
- Fixed bug: when n_sentences_before or n_sentences_after were 0, it defaulted to 2 instead

* post make lint

* changed API parameters to CamelCase to make consistent; added MMR

* adding support for core indexing with Vectara

* removed debug msg

* updated timeout

* made tests all pass