feat: allow specifying noindex for robots

[AlliotTech]

PR Checklist

• [x] The changes have been tested (for bug fixes / features).
• [x] Docs in NexT website have been added / updated (for features).(ref: https://github.com/next-theme/theme-next-docs/pull/131)

PR Type

• [ ] Bugfix.
• [x] Feature.
• [ ] Improvement.
• [ ] Code style update (e.g. formatting, linting).
• [ ] Refactoring (no changes to functionality and APIs).
• [ ] Documentation.
• [ ] Translation.
• [ ] Other... Please describe:

What is the current behavior?

Issue resolved:

What is the new behavior?

• allow add noindex head meta.(ref: block-indexing)

How to use?

Add in page frontmatter:

noindex: true