ci: add preview github action for specs

[rootulp]

We should add the preview github action we have on the docs https://github.com/celestiaorg/docs/blob/main/.github/workflows/preview.yml

This allows us to verify the deployed site on the pull request.

Fine for this to be a follow up.

Originally posted by @MSevey in #796 (comment)

[rootulp]

We added the workflow in #1497 but there is a permissions issue that prevents the action from commenting on the PR with a link to the deployed GH pages.

[rootulp]

PR preview works on branches for this repo. See #1501 (comment). However, PR preview doesn't work for branches on forks.

This Action does not currently support deploying previews for PRs from forks, but will do so in the upcoming v2.

Ref: https://github.com/rossjrw/pr-preview-action so I'll create a separate issue to track upgrading the PR Preview action when v2 is released.