Generate index.html without re-scanning repositories

The `satis build` command does a couple of things:
1. Scans repositories for new versions.
2. Generates a `packages.json` file.
3. Generates an `index.html` file.

When tweaking the Twig template for `index.html`, it would be useful to have a command for only doing step 3, or at least to avoid doing step 1, which can take a long time.

Start with the `satis build` command and trace how it scans repositories, generates `packages.json`, and renders `index.html` from the Twig template. Determine where the command can skip repository scanning while still regenerating the index, then verify that template changes update `index.html` without performing step 1.

Written by the indexing model from the issue text.