Allow to navigate from the live preview to the PR

[tbouffard]

IMPORTANT: this requires changes in https://github.com/bonitasoft/bonita-documentation-theme

Currently, the title of the preview references the Pull Request but there is no way to navigate to this PR. We have icon to see the AsiiDoc source of a page, edit it, access to the contribution guide but no link.

The link could be created on the "PR subtitle"

The link could be computed by the GH action and pass as an argument of the build preview script. This url is probably already available in the pull_request event (in the github context). See https://docs.github.com/en/webhooks-and-events/webhooks/webhook-events-and-payloads#pull_request
Then a new AsciiDoc attribute or an Antora site key could be set and use by the bonita theme to build the link.