Automate post publishing

[jean-michelet]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the issue has not already been raised

Issue

We need to investigate a strategy to automate publication.

@Eomm suggested:

1. someone sent a PR
2. the PR is merged
3. a cron GHA check if there are unpublished posts and tries to publish the first one - after do it it commit a simple publishedAt datestamp

We can start by doing this manually I guess.