Chart readme standalone page

[momesgin]

Summary

Fixes #14919

Added a "View separately" action on chart detail readme that opens a standalone, auth-protected readme page in a new tab.

Occurred changes and/or fixed issues

• Added readme route: /c/:cluster/readme with requiresAuthentication: true.
• Added standalone readme page to render chart readme content only.
• Added chart detail readme action button (hover/focus visible) to open standalone readme.
• Added readme URL helper for route/query generation.
• Added i18n entries for readme action label/tooltip.
• Updated interaction styling for button visibility and pointer behavior.

Technical notes summary

• Route uses existing auth guard behavior via requiresAuthentication.
• Added helper + unit coverage for URL generation and readme open behavior.
• Added readme page tests for query parsing and fetch flow.

Areas or cases that should be tested

• Chart detail: button visibility on hover/focus and click behavior.
• New-tab readme: content-only layout, no side nav/header.
• Auth: unauthenticated access to /c/:cluster/readme.
• Theme: light/dark rendering in standalone readme.
• Keyboard navigations.

Areas which could experience regressions

• Chart detail readme action area (hover/focus/click behavior).
• Query-based chart context propagation to readme page.
• Standalone layout/theming behavior.
• Keyboard navigations.

Screenshot/Video

chrmstln.mov

Checklist

• The PR is linked to an issue and the linked issue has a Milestone, or no issue is needed
• The PR has a Milestone
• The PR template has been filled out
• The PR has been self reviewed
• The PR has a reviewer assigned
• The PR has automated tests or clear instructions for manual tests and the linked issue has appropriate QA labels, or tests are not needed
• The PR has reviewed with UX and tested in light and dark mode, or there are no UX changes
• The PR has been reviewed in terms of Accessibility
• The PR has considered, and if applicable tested with, the three Global Roles Admin, Standard User and User Base