ICU-22922 migrate download index

[markusicu]

Migrate https://icu.unicode.org/download to GitHub Pages, second in the nav_order.
Small updates to download/76.

The merge target is main, not maint/maint-76, because we want the changes to go live right away in GitHub Pages. This is why I am using the ICU 77rc BRS ticket.

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22922
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• 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. Example: "ICU-1234 Fix xyz"
• 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]

LGTM

[sven-oly]

Yay!

[jira-pull-request-webhook]

Hooray! The files in the branch are the same across the force-push. 😃

~ Your Friendly Jira-GitHub PR Checker Bot