doc: Tell about HdyFold being removed

This also details how the HdyLeaflet API is affected.
11 jobs for drop-hdy-fold in 15 minutes and 47 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #241066
librem5
api-visibility

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passed #241068
librem5
build-debian-clang

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passed #241067
librem5
build-debian-gcc

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passed #241069
librem5
build-fedora-gcc

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  Test+Docs
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librem5
build-gtkdoc

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passed #241071
librem5
unit-test

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43.7%
 
  Package
passed #241072
librem5
package-debian-buster

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passed #241073
librem5:arm64
package-debian-buster:arm64

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passed #241074
flatpak
package-flatpak

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  Test Package
passed #241075
librem5
autopkgtest-debian-buster-package

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passed #241076
librem5
lintian-debian-buster-package

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