Update calibration docs

[znicholls]

Description

Checklist

Please confirm that this pull request has done the following:

• (N/A) Tests added
• Update scmdata calibration docs
• Split out emcee plotting functions
• Documentation added (where applicable)
• Changelog item added to changelog/

[codecov]

Codecov Report

Attention: Patch coverage is 59.75610% with 33 lines in your changes missing coverage. Please review.

Project coverage is 92.93%. Comparing base (2d589a0) to head (95b748f).
Report is 23 commits behind head on main.

Files with missing lines	Patch %	Lines
src/openscm_calibration/emcee_utils.py	51.61%	30 Missing ⚠️
src/openscm_calibration/cost/scmdata.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
- Coverage   98.60%   92.93%   -5.68%     
==========================================
  Files          16       15       -1     
  Lines         574      580       +6     
  Branches       79       78       -1     
==========================================
- Hits          566      539      -27     
- Misses          7       40      +33     
  Partials        1        1              

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