Change product landing page introLinks to a general map of link titles to links (#26360)

* Add cta button to frontmatter and context

* Add cta to Page

* Render cta button in product landing hero

* Handle external links

* Add simple unit test for cta link

* Address feedback

Co-authored-by: Peter Bengtsson <mail@peterbe.com>

* Actually push condition update

* Show it's an external link

* Refactor FullLink so we use Link once

Co-authored-by: Peter Bengtsson <mail@peterbe.com>

* Custom link can also be null

* Rename 'cta' to 'custom' and make it the last introLink

* Update tests with 'cta' to 'custom' change

* Filter once

* Revert "Filter once"

This reverts commit a3f9a8a06b505d77fed47ca96a66c187c86c3c91.

* Update introLinks to a map of titles and URLs

* No more custom introLink in LandingHero

* Simplify introLinks processing

* introLinks can also be null

Co-authored-by: Peter Bengtsson <mail@peterbe.com>

* more concise

Co-authored-by: Peter Bengtsson <mail@peterbe.com>

* No longer necessary with the a plain introLinks map

Co-authored-by: Peter Bengtsson <mail@peterbe.com>

* '.entries()` is simpler

Co-authored-by: Peter Bengtsson <mail@peterbe.com>

* 'link' could be false depending on what version you're on

* Update test for new introLinks

Co-authored-by: Peter Bengtsson <mail@peterbe.com>

lib/frontmatter.js

@@ -77,11 +77,6 @@ export const schema = {
     },
     introLinks: {
       type: 'object',
-      properties: {
-        quickstart: { type: 'string' },
-        reference: { type: 'string' },
-        overview: { type: 'string' },
-      },
     },
     authors: {
       type: 'array',