An offline docs bundle, and an air-gapped guide #164

Part of ComlineProject/core#227. Proposal: #163, proposals/offline/supply.md#tooling-into-the-gap.

• Build the documentation as a self-contained archive: no Google Fonts or other outside assets (theme font: false or self-hosted fonts, mkdocs-material's offline plugin), attached to releases.
• Once the features land, add a guide to using Comline in an air-gapped network (fetch, vendor, mirrors, bundles, installing the tooling).