Feature request to support djot

Issue #79 is getting crowded and hard to use. This issue is to make the request for [**djot**](https://djot.net/) support easy to follow.

Requested twice: https://github.com/Automattic/harper/issues/79#issuecomment-2330717766 and https://github.com/Automattic/harper/issues/79#issuecomment-3368735923

Start by reading issue #79 and the linked djot specification to understand the existing request and the format to support. Done means Harper supports djot as requested, but this issue does not name implementation files or tests, so the relevant entry points and acceptance checks must be identified first.

Written by the indexing model from the issue text.