Simple app to add ActivityPub comments

We have started using [GisCus](https://giscus.app) on the ActivityPods blog (see [here](https://activitypods.org/the-road-to-activitypods-2-0/#inject-comments)). It's a simple app which adds an iframe, load Github discussions comments, and allow to post new ones. All this through Github API.

I'm thinking that we could develop a similar app for ActivityPub comments, which could be used in Astro-generated websites, but also anywhere else.

## Implementation

The iframe would load an URL like https://activitypubcomments.app/{URI_OF_THE_RESOURCE}

It would load the comments in the `replies` of the linked resource.

## Issues

- ActivityPub doesn't have a standard auth method, so how do users authenticate ? We could imagine different auth method, like with a Mastodon account, or with an ActivityPods (soon Solid-OIDC) account.
- To what ActivityPub account will we post the comments ? Posting to our outbox is easy once authentication works, but if there is no recipients, our comments will be left in the void.

Start by reviewing the linked GisCus example and the proposed iframe URL pattern. Define how comments are read from a resource's replies, how users authenticate, and which ActivityPub account receives posts; the issue does not yet establish decisions or acceptance criteria for done.

Written by the indexing model from the issue text.