[docs] Add stub page for Markdown release notes

[colleenmcginnis]

Related to #9440

Adds a stub page for the Elastic Agent Markdown release notes to prevent failing docs checks on main while we wait for #9440 to be reviewed/merged.

[mergify]

This pull request does not have a backport label. Could you fix it @colleenmcginnis? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-./d./d is the label that automatically backports to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[github-actions]

🔍 Preview links for changed docs

• docs/release-notes/index.md

[elasticmachine]

Pinging @elastic/ingest-docs (Team:Docs)