Update yelp-xsl to 3.20.1

3.20.1
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* Updated translations:
  fur  (Fabio Tomat)

3.20.0
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* Updated translations:
  da    (Ask Hjorth Larsen)
  oc    (Cedric Valmary)

3.19.90
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* Move linktrails stubs to common for all source formats to use
* Allow Mallard example to be a formal element with title
* Added html.js.script template for easier site overrides
* Don't force xref attributes on links in Mallard cache
* Updated translations:
  et    (Piotr Drag)
  hi    (Piotr Drag)
  is    (Sveinn i Felli)
  km    (Piotr Drag)
  pt    (Piotr Drag)
  si    (Piotr Drag)
  sl    (Piotr Drag)

(prlw1)