Summary

• update the README variable font links to point at a direct reference page
• leave the existing Inter features link unchanged

Validation

• curl -L --max-time 20 https://rsms.me/inter/#variable returned HTTP 200, but the fetched page had no id="variable" or name="variable" fragment target
• curl -L --max-time 20 https://en.wikipedia.org/wiki/Variable_font returned HTTP 200 with title Variable font - Wikipedia
• rg -n "rsms\.me/inter/#variable|en\.wikipedia\.org/wiki/Variable_font|OpenType features" README.md -S
• git diff --check

No build tests were run because this only updates README links.

Addresses #784.