Add archive redirects for Calico Enterprise 3.20 and 3.19

CE 3.20 was archived (dropped from onlyIncludeVersions in
docusaurus.config.js and repointed to archive-ce-3-20.netlify.app in
archive.md), but the _redirects splat rule was never added, so
/calico-enterprise/3.20/* returned 404 on docs.tigera.io. CE 3.19 had
the same gap.

Add the Netlify archive splat rules for both versions, matching the
existing pattern for 3.14-3.18, and drop the now-stale bare
/calico-enterprise/3.20/ redirect (which pointed to a path that is no
longer built).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ctauchen

static/_redirects

@@ -43,7 +43,6 @@
 /calicc-enterprise/3.23/ /calico-enterprise/latest/about 301
 /calico-enterprise/3.22/ /calico-enterprise/3.22/about 301
 /calico-enterprise/3.21/ /calico-enterprise/3.21/about 301
-/calico-enterprise/3.20/ /calico-enterprise/3.20/about 301
 /calico-enterprise/3.18/ /calico-enterprise/3.18/about 301
 /calico-enterprise/3.17/ /calico-enterprise/3.17/about 301
 /calico-enterprise/3.16/ /calico-enterprise/3.16/about-calico-enterprise 301
@@ -56,6 +55,8 @@
 /calico/3.26/* https://archive-os-3-26.netlify.app/calico/3.26/:splat 301
 /calico/3.25/* https://archive-os-3-25.netlify.app/calico/3.25/:splat 301
 /calico/3.24/* https://archive-os-3-24.netlify.app/calico/3.24/:splat 301
+/calico-enterprise/3.20/* https://archive-ce-3-20.netlify.app/calico-enterprise/3.20/:splat 301
+/calico-enterprise/3.19/* https://archive-ce-3-19.netlify.app/calico-enterprise/3.19/:splat 301
 /calico-enterprise/3.18/* https://archive-ce-3-18.netlify.app/calico-enterprise/3.18/:splat 301
 /calico-enterprise/3.17/* https://archive-ce-3-17.netlify.app/calico-enterprise/3.17/:splat 301
 /calico-enterprise/3.16/* https://archive-ce-3-16.netlify.app/calico-enterprise/3.16/:splat 301