Simplify documentation CI

[ValerianRey]

• Make build-deploy-docs.yml not trigger on PR
• Change Build_and_deploy_doc to run in the prod-documentation environment
• Change Build_and_deploy_doc job name
• Remove cleanup-pr-docs.yml
• Revert change in CONTRIBUTING.md explaining how to preview PRs
• Rename main folder to latest

This PR removes support for documentation build preview in PRs, because we cannot have a satisfying solution with GitHub Pages that will also work with forks.

It also fixes a security issue of having the DOCUMENTATION_DEPLOY_KEY be a repository-wide secret not protected by any environment. To avoid this, it uses instead the new PROD_DOCUMENTATION_DEPLOY_KEY, which is an environment secret of the new prod-documentation environment, tied to the main branch or the version tags. This private key matches the new public deploy key PROD_DOCUMENTATION_DEPLOY_KEY of TorchJD/documentation, that has write access to it.

Lastly, this PR improves the name of the job by using the dedicated name field and renames the main folder to latest.

Short-term future steps after merging this:

• Remove the DOCUMENTATION_DEPLOY_KEY private key from TorchJD/torchjd and the associated public key from TorchJD/documentation.
• Manually remove existing pr/* folders in TorchJD/documentation.
• Manually remove the existing main folder in TorchJD/documentation.

Long-term future steps:

• Add support for PR preview whenever GitHub Pages supports this, or by switching to readthedocs or another tool supporting this. Since this is quite a bit of work and possibly comes with a monthly subscription (like 5$/month for readthedocs), we will only do this when it becomes much more important.

[codecov]

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All modified and coverable lines are covered by tests ✅

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[PierreQuinton]

LGTM. Just wondering, should we change deploy to main to deploy to latest? If so we would have latest, stable and one for each version.