Add a public API endpoint and documentation to trigger Save Code Now from webhook

Idea from @rdicosmo:

As an alternative to the "save to SWH" github action, we could provide an API endpoint that can be called by forges (github, gitlab, gitea/gogs, ...) to notify us of a push or new tag.

Motivation:

1. less wasteful on the forge's side (it's just an HTTPS request, not a container with its own nodejs process, ...)
2. easier to setup for users (a few clicks in the forge's UI)
3. more secure (no need to trust the action's author)
4. works for all forges
5. might allow us to extract information from the webhook payload, in the future
6. ...

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Migrated from T4548 (view on Phabricator)