Add creating-an-ingest-workflow tutorial

Based on contents of the initial draft of the tutorial
https://docs.google.com/document/d/1_16VYT5MU8oXJ4t6HUHp_smx_kgF9OMsORSCldee1_0/edit#heading=h.r95jmyuit0s0

Split out the steps to get the uncurated NCBI Dataset data into a
snippet that can be shared between the two ingest tutorials.

src/snippets/uncurated-ncbi-dataset.rst

@@ -0,0 +1,11 @@
+.. code-block::
+
+    $ nextstrain shell .
+    $ datasets download virus genome taxon <taxon-id> --filename ingest/data/ncbi_dataset.zip
+    $ dataformat tsv virus-genome --package ingest/data/ncbi_dataset.zip > ingest/data/raw_metadata.tsv
+    $ exit
+
+The produced ``ingest/data/raw_metadata.tsv`` will contain all of the fields available from NCBI Datasets.
+Note that the headers in this file use the human readable ``Name`` of the
+`NCBI Datasets' available fields <https://www.ncbi.nlm.nih.gov/datasets/docs/v2/reference-docs/command-line/dataformat/tsv/dataformat_tsv_virus-genome/#fields>`_,
+while the pipeline uses the config's ``curate.field_map`` dictionary to convert these to computer friendly ``Mnemonic``.