utility to export gnoweb content as static HTML files

[moul]

We need a utility that can natively export the content of a blockchain as clickable HTML files. This command will be used in CI to export the content of a blockchain, which can then be published on Netlify or any other service that exposes static files for each PR.

Proposed Solution

The utility should be simple to run, making CI on external repositories a no-brainer. Ideally, it should be integrated into gnodev so that:

• gnodev serves the content.
• gnodev --export-html exports everything as HTML files.

This way, it's a single command to run in CI, making the process straightforward and efficient.

cc @gfanton @leohhhn