fix: update routing form identifier hint text with learn more link

[CarinaWolli]

What does this PR do?

Updates the routing form identifier field's info text from "Used for URL parameters, like &name=john" to a more descriptive message with a "Learn more" link pointing to the help article on connecting routing form data to booking questions.

Before:

After:

Changes:

• Updated the identifier_url_parameter_hint locale string in en/common.json
• Widened hint prop type on InputFieldProps from string to React.ReactNode so JSX can be passed
• Replaced the plain text hint with a LearnMoreLink component (same component already used in advanced event type settings) that links to https://cal.com/help/routing/connect-routing-form-to-booking-questions

The LearnMoreLink component already handles self-hosted instances by stripping the link and showing only the text.

Reviewer Notes

• The hint type widening from string → React.ReactNode is backward-compatible (string is a valid ReactNode), and TextField.tsx renders {hint} directly in JSX, so no breakage expected. Worth a quick sanity check that no other consumer relies on hint being strictly a string.
• Only the English locale string is updated. Other locales will continue showing their existing text (without the "Learn more" link) until translated—this is the standard i18n workflow.
• No visual demo included—reviewer should verify the hint renders correctly with the underlined link styling.

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. N/A
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

How should this be tested?

1. Go to any routing form → edit or create a form field
2. Look at the "Identifier" field's hint text below the input
3. Confirm it reads: "For URL parameters that can be used to prefill booking questions, e.g. &name=john. Learn more"
4. Confirm "Learn more" is an underlined link opening https://cal.com/help/routing/connect-routing-form-to-booking-questions in a new tab
5. On self-hosted (non-cal.com) instances, the "Learn more" link should not appear—only the text

Link to Devin run: https://app.devin.ai/sessions/ff20e885baab462fb304ec871a9975b1
Requested by: @CarinaWolli

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[cubic-dev-ai]

No issues found across 3 files

[supalarry]

Looks good but have comment on type issue:

[github-actions]

E2E results are ready!

• Workflow #104191.2 latest results

#27837 (comment)