Pre-render static pages to make them indexable by search engines

Currently only the main page of the export is pre-rendered (and correspondingly indexed by Google. to use Logseq docs as an example: https://www.google.com/search?q=site%253Ahttps%253A%252F%252Fdocs.logseq.com)

The fact that there is at least one page pre-rendered is exciting because it implies that infrastructure for it is already there.
It'd be great if there were an option to pre-render all the pages (optionally - only pages tagged with a particular tag)

With many pages actual pre-rendering may take some time but:
- After initial build - it can be done incrementally only for changed pages, which should take minimal time
- You can make it opt-in - I imagine people who value SEO/searchability of the website would be happy to take increase build time as a tradeoff

Start by tracing the existing pre-rendering infrastructure in the publish action and CLI described in the issue. Define an opt-in path for rendering all pages or pages with a selected tag, then verify that unchanged pages can be skipped on later builds and that the generated pages are indexable by search engines.

Written by the indexing model from the issue text.