fix(ui-billboard): point icons doc links at valid icons pages

The Billboard READMEs linked the hero icon docs to `icons-react`, which
is not a route in the docs app. Point v2 at `icons` and v1 at
`legacy-icons`, and remove a now-inaccurate note from the Icons page.

INSTUI-5121

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: git-nandor

packages/__docs__/src/Icons/index.tsx

@@ -49,8 +49,7 @@ const IconsPage = () => {
         }
       >
         <Alert variant="info" margin="0 0 medium">
-          The version selector does not affect this page. For icons compatible
-          with older versions, use{' '}
+          For icons compatible with older versions, use{' '}
           <Link
             href="legacy-icons"
             onClick={(e) => {

packages/ui-billboard/src/Billboard/v1/README.md

@@ -26,7 +26,7 @@ type: example
   don't pass interactive content to the `message` prop if you have set the `href`
   or `onClick` props).
 - Use the `size` prop to adjust the size of the icon and text.
-- Pass [Instructure icons](icons-react) to the `hero` property via a function
+- Pass [Instructure icons](legacy-icons) to the `hero` property via a function
   _(see examples)_, and they will be sized correctly based on the Billboard's
   `size`.
 

packages/ui-billboard/src/Billboard/v2/README.md

@@ -26,7 +26,7 @@ type: example
   don't pass interactive content to the `message` prop if you have set the `href`
   or `onClick` props).
 - Use the `size` prop to adjust the size of the icon and text.
-- Pass [Instructure icons](icons-react) to the `hero` property via a function
+- Pass [Instructure icons](icons) to the `hero` property via a function
   _(see examples)_, and they will be sized correctly based on the Billboard's
   `size`.