ICU-21814 Fixed a bunch of broken links

[MrCheeze96]

Most of the links on this page were broken. I've tried to see what the links was referring to and updated the links to what I found out to be most logical. If there is any link that doesn't go to the intended page, please feel free to change it to what it should be! :)

Checklist

• [x] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-21814
• [x] Required: The PR title must be prefixed with a JIRA Issue number.
• [x] 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.
• [x] Issue accepted (done by Technical Committee after discussion)
• [ ] Tests included, if applicable
• [x] API docs and/or User Guide docs changed or added, if applicable

[CLAassistant]

All committers have signed the CLA.

[markusicu]

After tomorrow, this will need to target ICU 72 via ticket ICU-21959.

[markusicu]

After tomorrow, this will need to target ICU 72 via ticket ICU-21959.

ICU 72 is nearing code freeze...