astradb: bootstrapping Astra DB as Partner Package (#16875)

**Description:** This PR introduces a new "Astra DB" Partner Package.

So far only the vector store class is _duplicated_ there, all others
following once this is validated and established.

Along with the move to separate package, incidentally, the class name
will change `AstraDB` => `AstraDBVectorStore`.

The strategy has been to duplicate the module (with prospected removal
from community at LangChain 0.2). Until then, the code will be kept in
sync with minimal, known differences (there is a makefile target to
automate drift control. Out of convenience with this check, the
community package has a class `AstraDBVectorStore` aliased to `AstraDB`
at the end of the module).

With this PR several bugfixes and improvement come to the vector store,
as well as a reshuffling of the doc pages/notebooks (Astra and
Cassandra) to align with the move to a separate package.

**Dependencies:** A brand new pyproject.toml in the new package, no
changes otherwise.

**Twitter handle:** `@rsprrs`

---------

Co-authored-by: Christophe Bornet <cbornet@hotmail.com>
Co-authored-by: Erick Friis <erick@langchain.dev>

libs/community/langchain_community/vectorstores/astradb.py

@@ -20,6 +20,7 @@ from typing import (
 )
 
 import numpy as np
+from langchain_core._api.deprecation import deprecated
 from langchain_core.documents import Document
 from langchain_core.embeddings import Embeddings
 from langchain_core.runnables import run_in_executor
@@ -61,6 +62,11 @@ def _unique_list(lst: List[T], key: Callable[[T], U]) -> List[T]:
     return new_lst
 
 
+@deprecated(
+    since="0.1.23",
+    removal="0.2.0",
+    alternative_import="langchain_astradb.AstraDBVectorStore",
+)
 class AstraDB(VectorStore):
     """Wrapper around DataStax Astra DB for vector-store workloads.