Use official readthedocs action for PR preview links

[brichet]

For the record, we should restore the Jupyterlite preview in the PR comment when available.

• the preview has been removed in Remove (temporary) the lite preview from the PR comment #290
• it needs Support replacing multiple instances of {docs-pr-index-url} in message-template readthedocs/actions#47
• it has been discussed in Fix typo in ReadTheDocs PR link automation #285 (comment)

[martinRenou]

We have a preview with rtd now, it works more reliably than the app.sharing app we had before.

[martinRenou]

We could add back a message in the PR with direct link to the lite preview though, as it requires an extra click to access it right now :D

[mfisher87]

Fixed here: #356 While we wait for the upstream action to accept the change, we're using a fork that supports the feature we needed :)

Please re-open if I misunderstood the need here :)

[brichet]

We should probably keep this issue opened, to restore action on https://github.com/readthedocs/actions when the update is available upstream.

[mfisher87]

Sounds good!