Fix some links in documentation

[hugovdm]

WIP: maybe this should become part of ICU-22438 ?

• Skip over the manual redirects that are icu4[cj]/readme.html.
• Update links: site.icu-project.org/processes/contribute is no more.
• Update link paths: some relative links in moved docs were pointing at userguide/icu4c/icu4c
• Add .html suffixes, letting unicode-org.github.io sidebar correctly show which page is being viewed. However it would also break Markdown navigation. Which use case are we optimising for? GitHub Pages, or Markdown source?

I wasn't very thorough with my searches, this was based on broken links

rgrep processes/contribute . rgrep 'icu4[cj]/readme.html' . egrep -r '/packaging)|/build)' .

I've not fixed all the /icudata.md links yet for example. Should perhaps add all of those to this PR?

Checklist

• [ ] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-_____
• [ ] Required: The PR title must be prefixed with a JIRA Issue number.
• [ ] Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• [ ] Required: Each commit message must be prefixed with a JIRA Issue number.
• [ ] Issue accepted (done by Technical Committee after discussion)
• [ ] Tests included, if applicable
• [ ] API docs and/or User Guide docs changed or added, if applicable

[echeran]

FYI: You no longer need to ensure the .html suffix to get the left hand navigation bar to highlight the page correctly on page load, thanks to https://github.com/just-the-docs/just-the-docs/issues/1371.