Declare linkable parameters and transforms [doc-build]

[philippjfr]

Allows parameter links between widgets and Panel objects to automatically become jslinks when embedding the output.

Implements #1003

[codecov]

Codecov Report

Merging #1007 into master will increase coverage by 0.33%.
The diff coverage is 95.12%.

@@            Coverage Diff             @@
##           master    #1007      +/-   ##
==========================================
+ Coverage   85.88%   86.21%   +0.33%     
==========================================
  Files          99      100       +1     
  Lines       11325    11772     +447     
==========================================
+ Hits         9726    10149     +423     
- Misses       1599     1623      +24

Impacted Files	Coverage Δ
panel/widgets/misc.py	86.07% <ø> (ø)	⬆️
panel/pane/holoviews.py	84.55% <100%> (ø)	⬆️
panel/layout.py	83.89% <100%> (+0.15%)	⬆️
panel/pane/image.py	80.14% <100%> (+0.14%)	⬆️
panel/pane/plot.py	73.86% <100%> (ø)	⬆️
panel/widgets/select.py	94.3% <100%> (+0.06%)	⬆️
panel/widgets/ace.py	78.26% <100%> (+0.98%)	⬆️
panel/pane/markup.py	97.05% <100%> (ø)	⬆️
panel/pane/streamz.py	66.66% <100%> (+0.95%)	⬆️
panel/pane/plotly.py	88.88% <100%> (ø)	⬆️
... and 25 more

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