Bump jekyll from 3.6.3 to 4.3.4 in /docs/_spec #1028

Job	Run time
test_non_bootstrapped	0s
test	1s
test_scala2_library_tasty	0s
test_windows_fast	1s
MiMa	1s
test_windows_full	0s
community_build_a	1s
community_build_b	1s
community_build_c	1s
test_sbt	1s
test_java8	0s
build-msi-package	0s
build-sdk-package / build	8m 1s
build-chocolatey-package / build	15s
test-chocolatey-package	0s
publish_nightly	0s
publish_release	0s
nightly_documentation	0s
open_issue_on_failure	0s
	8m 23s

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