Add documentation of MESSAGE-MACRO tuning

[khaeru]

Edited from original text by @OFR-IIASA.

Also, similar to iiasa/ixmp#281, the docs tree is 'flattened' so that more pages are visible at the first level in the sidebar, grouped under explanatory headings/captions.

How to review

• Look at the built documentation: https://message.iiasa.ac.at/en/mm-document/macro.html

PR checklist

• Tests added. No code changes
• Documentation added.
• Release notes updated.

[codecov]

Codecov Report

Merging #345 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #345   +/-   ##
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  Coverage   88.80%   88.80%           
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  Files          29       29           
  Lines        2555     2555           
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  Hits         2269     2269           
  Misses        286      286           

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[khaeru]

@OFR-IIASA I've requested review from you particular but FYI @behnam-zakeri @danielhuppmann @volker-krey per e-mail yesterday, the URL in the description above points to a new page with the text from the e-mail.

I've made some edits for order and context, so please approve if the description is still accurate and faithful to the original instructions.

[OFR-IIASA]

Thanks for creating this PR Paul from the intial text supplied. I have no further comments; looks good to me.