CI: consolidate docs deployment workflows and add PR previews

[AritraDey-Dev]

This PR consolidates docs deployment into a single workflow with automatic PR previews and Github pages deployment.

Fixes #121

/cc @dlebauer

To pass the CI check need to follow step 4 mentioned in the issue i.e.

Once the PR is submitted or ready to submit, the following settings need to be changed by a maintainer
change build and deploy from 'deploy from a branch' to 'GitHub actions' (settings --> pages --> build and deploy)
Make sure the following are enabled
"Read and write permissions" for GITHUB_TOKEN (settings -> actions -> general --> workflow permissions
"Allow GitHub Actions to deploy to GitHub Pages” (settings --> actions --> general --> workflow permissions
Require status check for the job.

[dlebauer]

@AritraDey-Dev thanks for taking on this task, pr-previews will be a useful feature! I have updated the repository settings as indicated.

One change needed - right now the PR preview runs for all PRs, including forks, but the job fails because the action doesn't have permissions to push from the fork.

Please one of the following:

1. (quickest, no previews from forks) don't allow builds on PRs from forks. This isn't ideal, but changing line 38 to if: github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name == github.repository might work
2. to enable previews from forks we could use a two-step process
   • pull_request workflow just builds sites and uploads it as an artifact
   • a separate script that can be triggered by a maintainer (workflow_dispatch) downloads the artifact and deploys it to a dedicated branch using GITHUB_TOKEN
3. There may be other and possibly better ways to achieve the pr-previews.