release-73-1
ICU-21304 Update old userguide links to unicode-org.github.io
ICU-22220 Add step for updating API Doc table
ICU-22285 how to include gb2312 & big5han collation tailorings
ICU-20088 User Guide migration: Add navigation structure and TOC for Jekyll, fix formatting issues, etc.
ICU-22194 Add CI job to generate Github Pages using Github Actions
ICU-21310 replacement User Guide main index page
ICU-21580 change site.icu-project.org to icu.unicode.org etc