docs: add release availability information

[MikeMcC399]

Description

Adds a new section "Release Availability" to the README document.

It explains what influences when images are available on Docker Hub and

• describes how image tags can first appear without the related OS/ARCH images
• explains why Alpine images may not initially be available for security releases

Motivation and Context

• relates to issue Document and implement strategy when musl releases (for Alpine) are missing #2363

It clarifies what can be expected of the publication process and avoids users interpreting certain delays
as error conditions, when they are inherent to the process flow.

Testing Details

The documentation should correspond to how things work, which has been observed during the March 2026 security update, and the Node.js 25.9.0 update.

Example Output

N/A

Types of changes

• Documentation
• Version change (Update, remove or add more Node.js versions)
• Variant change (Update, remove or add more variants, or versions of variants)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other (none of the above)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• All new and existing tests passed.

[mcollina]

lgtm