ICU-22533 move custom normalization page from Sites to GitHub

[markusicu]

Move https://icu.unicode.org/design/normalization/custom
to GitHub Pages markdown at docs/design/normalization/custom.md

Started with https://codebeautify.org/html-to-markdown
followed by fixing lists, headings, links, some code style.

Copied the two tables from the HTML source view of the Sites page, unescaped their HTML, and fiddled a little with the style. Could probably use more work to minimize cell padding/spacing, and we might not need explicit heights and widths -- but I think it's ok for now.

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22533
• 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]

LGTM.

In the PR preview view, it looks like Github's markdown rendering output might override the table styling with its own styles. This might be orthogonal to how Jekyll deals with things (no overrides) as its generates HTML that gets ultimately served up as-is on the User Guide site.

[markusicu]

Thanks, merged. The zero-padding of the tables didn't make it to https://unicode-org.github.io/icu/design/normalization/custom.html :-/

I added the "moved to" link to https://icu.unicode.org/design/normalization/custom