Add footnotes

[theletterf]

This PR adds footnotes using MyST's notation.

---

GenAI disclosure: I used Cursor 2 with Claude Sonnet 4.5, Claude Opus 4.5, and Gemini 3 Pro.

[github-actions]

🔍 Preview links for changed docs

• docs/syntax/footnotes.md
• docs/syntax/frontmatter.md

[theletterf]

@Mpdreamz May I merge?

[Mpdreamz]

If either @itsalexcm @reakaleek and @cotti are happy always, let alone all three 😸

[Mpdreamz]

I do think callouts to the release notes should be bigger/more distinct?

They have a very small mouse-click area:

[itsalexcm]

Suggestion to expand the click area (which is a bit small, yes). First, put the "." before the sub. Then:

.markdown-content a.footnote-ref {
    margin-right: var(--spacing);
    padding: 0 var(--spacing);
}

Visual example:

[theletterf]

@Mpdreamz With @itsalexcm blessing, you can check the new improved clickable area added in 80388cf here:

https://docs-v3-preview.elastic.dev/elastic/docs-builder/pull/2275/syntax/footnotes