Add docs for userdefined layout

[randallfrank]

Added documentation for the userdefined layout and update plot attributes.

[randallfrank]

Notes:
The XML is reformatted, but I hand-added the additional plot types (heatmap, parallel, sankey) and I added the two new properties used to set min/max values on parallel coordinate scales.

The userdefined link is not there yet, but all the others work and point to the s3 'is' content.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (970ccf0) 75.76% compared to head (80d9eaa) 75.76%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #83   +/-   ##
=======================================
  Coverage   75.76%   75.76%           
=======================================
  Files          16       16           
  Lines        4997     4997           
  Branches     1050     1050           
=======================================
  Hits         3786     3786           
  Misses        860      860           
  Partials      351      351           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.