Update docs to reflect new item_mip_data_processing repo

[khaeru]

This PR adjusts the documentation to describe the contents of the 'database' vs. 'item_mip_data_processing' vs. 'metadata' repos, per recent changes.

Docs preview on RTD

[codecov[bot]]

Codecov Report

Merging #12 into master will decrease coverage by 0.08%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
- Coverage   47.77%   47.68%   -0.09%     
==========================================
  Files          25       25              
  Lines        1147     1147              
==========================================
- Hits          548      547       -1     
- Misses        599      600       +1

Impacted Files	Coverage Δ
item/common.py	61.76% <0%> (-1.48%)	:arrow_down:

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[khaeru]

@pkyle can you please look quickly at the docs preview link above and ensure that the pages give a clear sense of what is in this repo vs. the new one?

If so, please leave an "Approve" review and I'll merge.

[khaeru]

Looks good to me--so the metadata repo is something you'll be maintaining, unrelated to the MIP3? The region metadata are generally the same, though not always, and the scenarios are different.

As described on Slack, I would prefer to have metadata for all projects live in that repo. For instance, I notice that the item_mip_data_processing has regions.yaml, regions_item2.yaml and regions_item3.yaml; it would be nice if all such files were there to be picked up when needed.

The R MIP code and Python historical data code can make use of those files in distinct ways, but they'll at least still be the same files.