feat(ingest): add basic chunking to ingest (#2380)
The new "basic" chunking strategy and overlap options need to be
available from the ingest CLI. An ingest test of those features is also
welcome, both to verify the ingest feature and to defend against
regressions in the chunking code.
Add a local ingest test exercising both the "basic" chunking strategy
and intra-chunk overlap. Since there is no new source connector
involved, use the local ingest source and destination. Update
documentation to suit, filling in some details that hadn't made it into
the docs yet.
