OCI journey - OIE upgrade: test customizations guide

[brentschaus-okta]

Description:

• What's changed?
  Adds a new OIE-upgrade journey guide, Test your widget's existing customizations in a test environment (/docs/guides/oie-upgrade-test-widget-custom/main/), which explains what to test, how to set up a test environment, and how to fix common issues when validating Sign-In Widget customizations (CSS, JavaScript hooks, i18n, branding, custom sign-in pages) before an Identity Engine production upgrade.

  This PR also:

  • Registers the guide in guides/index.md and the navbar.
  • Adds intra-guide links to Identity Engine, design tokens, Keep me signed in, and sign-in flow concepts/guides/help topics.
  • Adds inbound links to the new guide from four related non-journey docs: custom-widget, custom-widget-gen3, custom-widget-migration-gen3 (all in ## See also), and the ie-limitations Sign-In Widget upgrade section.
  • Renames the guide directory from test-oie-preview-widget-custom to oie-upgrade-test-widget-custom, and fixes the stale navbar guideName left by that rename.

  Note: navbar placement may need revisiting as the broader journey doc structure firms up. Some cross-references currently point to existing dev-docs guides as stand-ins for help-center topics that are still in progress (e.g., "Set up your test environment", the target is undefined support article).

• Is this PR related to a Monolith release?
  No. Targeting the July monthly developer docs release (code freeze June 26, 2026.07.0).

Resolves:

• OKTA-1185573

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[okta-prod-github-app]

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Article	Total score Required:80	Word and phrases (Brand, terms) Preferred: 80	Correctness (Spelling, grammar) Preferred: 80	Clarity (Readability) Preferred: 80	Inclusive language (+ accesibility) Preferred: 80
✅ packages/@okta/vuepress-site/docs/guides/custom-widget-gen3/main/index.md	86	91	50	84
✅ packages/@okta/vuepress-site/docs/guides/custom-widget-migration-gen3/main/index.md	90	75	75	100
✅ packages/@okta/vuepress-site/docs/guides/custom-widget/main/index.md	93	89	87	97
✅ packages/@okta/vuepress-site/docs/guides/ie-limitations/main/index.md	85	94	88	63
✅ packages/@okta/vuepress-site/docs/guides/index.md	87	67	80	78
✅ packages/@okta/vuepress-site/docs/guides/oie-upgrade-test-widget-custom/index.md	100	100	100	100
✅ packages/@okta/vuepress-site/docs/guides/oie-upgrade-test-widget-custom/main/index.md	88	89	84	80

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