fix(feedback): prevent crash submitting feedback without a highlight

[Jeremy Rose (jeremyrose-viam)]

Summary

• submitFeedback() in assets/js/feedback.js assumed a page element was always highlighted before submit, so highlightedInfo[0].text threw a TypeError when a user typed feedback and clicked Submit without highlighting anything (highlighting is optional).
• The uncaught error silently killed the click handler, so the button appeared to "not work" with no visible error to the user.
• Falls back to "(no element selected)" when nothing was highlighted.

Jira: DOCS-4968

Test plan

Note: We'll need to test this in production. CORS does not appear to be configured for dev or preview environments.

• Load a docs page, click "Give Feedback", type feedback text, click Submit without highlighting any page content — confirm it submits successfully instead of silently failing
• Confirm the existing flow (highlight an element, then submit) still works and includes the highlighted element's text

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