CTA: register default templates on docset.yml and toc.yml

[bmorelli25]

Background

The right-gutter CTA card can now be customized per page via cta frontmatter (see #3602).

Why is something more needed?

elastic/docs-content-internal#1475 requests adding a custom "Get started free" CTA and onboarding_token to all solution-specific pages under solutions/observability/, solutions/security/, and solutions/search. Doing this in frontmatter alone means editing ~600 files. Over time as new pages are added, folks will inevitably forget the frontmatter and we'll end up with an inconsistent mix of CTAs.

The proposed solution

Register a default CTA on the navigation file that owns a section. Templates still live in docset.yml; default_cta on docset.yml or a nested toc.yml selects which template applies to every page listed in that file.

# docset.yml
cta:
  observability:
    button:
      label: Get started free
      url: https://cloud.elastic.co/serverless-registration?onboarding_token=observability
toc:
  - toc: solutions/observability

# solutions/observability/toc.yml
default_cta: observability
toc:
  - file: index.md
  - file: apps/apm.md

Resolution order for a page:

1. An explicit, known cta frontmatter id (unchanged, always wins)
2. The default_cta registered for the page via its navigation file
3. The built-in trial default

This makes the docs-content side of #1475 a small change across docset.yml plus a few section toc.yml files. Pages that already carry cta frontmatter (e.g. the monitor-kubernetes / monitor-aws pages from elastic/docs-content#7135) keep their more specific CTA automatically.

Details: inheritance, edge cases, and one behavior change

• Navigation ownership. Defaults follow the nav tree, not URL path prefixes. New pages added to a section's toc.yml pick up its default_cta automatically.
• Nested overrides. A nested toc.yml can set its own default_cta; pages under it use that value instead of the parent navigation file's default.
• Conflicts are build errors. The same page registered with two different defaults, or a default_cta that doesn't match a template in docset.yml, fails the build rather than resolving silently.
• Behavior change for unknown frontmatter ids. Previously an unknown cta id warned and fell back straight to the default trial card. It still warns, but is now ignored, so a navigation default covering the page applies instead. The section CTA is more relevant than the generic card, and the warning still points at the typo.
• Isolated builds. A navigation-registered default resolves to a named, non-default template, so it renders in isolated builds too (same as explicit frontmatter) — contributors can preview it. The built-in default remains assembler-only.

Docs for the new key are added to docs/configure/content-set/cta.md and docs/configure/content-set/navigation.md.

Test plan

• 8 unit tests in ConfigurationFileCtaTests covering precedence, nested toc overrides, docset-level defaults, unknown template errors, and duplicate registration errors
• Manually verify a docset with default_cta on a section toc.yml renders the scoped CTA on pages in that nav file and the default elsewhere

[bmorelli25]

@elastic/docs-engineering I have this proposal for you. If it's too far from your vision LMK and we can close this.

[Mpdreamz]

drive by comment, would registering the default cta in toc.yml suffice?

[bmorelli25]

I've realigned this PR with your suggestion 👍