feat: optionally hide subpages

[jkomyno]

Optionally hide subpages via hide: true. The starter demo slides now reflect this new functionality.

Example:

---
src: ./subpage2.md
hide: true
---

In this example, ./subpage2.md is skipped. This:

• doesn't generate any extra slide
• doesn't forget about any other slide in the deck
• keeps $slidev.nav.currentPage and $slidev.nav.total consistent, as if the markdown block above was never defined in the first place.

Closes #720.

TODO:

• Highlight the new functionality in slidevjs/docs and slidevjs/docs-cn

[jkomyno]

Hey @antfu, thanks for merging this! I have a few new PRs incoming for slidev (both docs and Vercel config-related).
I wonder, are you interested in marking the repositories under the slidevjs organization with the hacktoberfest label, as in https://hacktoberfest.com/participation?