Add YAML-based prerender option to Markdown pages

[joanrieu]

Body

Summary

It should be possible to pre-render Markdown pages on an otherwise server-rendered website.

Background & Motivation

Markdown pages (.md files in the pages directory) do not currently have a yaml-based way to flag them as pre-renderable.

Considering that the server output mode disables pre-rendering by default and the export const prerender = true; statement in Astro files allows overriding it, it would make sense to support an equivalent prerender: true override setting in the YAML front-matter.

There is a precedent in providing an override flag for pages, as mentioned above for Astro files.

There is also a precedent in providing Astro-specific YAML front matter keys, as done with the (unrelated) layout: ... property in Markdown files.

Goals

• Offer a documented way to enable pre-rendering in a given Markdown file

Example

---
prerender: true
---

# Example

This is an always pre-rendered Markdown page.

[johnzanussi]

I ran into this issue today so I created an integration that enables prerender value in the frontmatter of Markdown files.

https://github.com/johnzanussi/astro-markdown-prerender