DOCS-404: add Gateway settings UI access guidance

[harrison-akeyless]

Summary

Adds clear instructions for how to access Gateway settings from the UI.

Changes

• Updated Configure Gateway landing page with a new Access Gateway Settings (UI) section
• Added a concrete example URL for the settings route
• Added cross-reference to Gateway Network Connectivity for port/deprecation context
• Added/updated a short connectivity note for the UI settings endpoint and Configure App deprecation behavior
• Replaced an empty section heading with Available Configuration Features

Validation

• markdownlint (file-scoped) passed for both edited files
• cspell (file-scoped) passed for both edited files
• lychee (file-scoped) reported no link errors

Tracking

• DOCS-404