Add support for publishing download site maintenance branch

[jcfr]

This branch is expected to be manually checkout on the server while the maintenance is taking place.

[slicerbot]

download.slicer.org preview [branch: deploy_download_maintenance_preview]

⚠️ The download preview is a static website generated using mock data ¹, is temporary and may be updated at anytime ²

Screenshot from https://deploy-download-maintenance-preview--slicer-org.netlify.app/download.html

Footnotes

1. See front matter variable download_mock associated with https://raw.githubusercontent.com/Slicer/slicer.org/main/download.markdown ↩

2. Due to limitation of Netlify preventing us from having multiple deploy previews associated with a single pull request and the impossibility of using repository secret in a workflow associated with a pull-request originating from forks, the deploy-download-preview site is only updated for pull request originating from this repository and will be overriden after another pull request is pushed or updated. ↩

[slicerbot]

download.slicer.org preview [branch: deploy_download_preview]

⚠️ The download preview is a static website generated using mock data ¹, is temporary and may be updated at anytime ²

Screenshot from https://deploy-download-preview--slicer-org.netlify.app/download_release_and_nightly.html	Screenshot from https://deploy-download-preview--slicer-org.netlify.app/download_only_release.html	Screenshot from https://deploy-download-preview--slicer-org.netlify.app/download_only_nightly.html
Screenshot from https://deploy-download-preview--slicer-org.netlify.app/download_release_and_nightly.html

Footnotes

1. See front matter variable download_mock associated with https://raw.githubusercontent.com/Slicer/slicer.org/main/download.markdown ↩

2. Due to limitation of Netlify preventing us from having multiple deploy previews associated with a single pull request and the impossibility of using repository secret in a workflow associated with a pull-request originating from forks, the deploy-download-preview site is only updated for pull request originating from this repository and will be overriden after another pull request is pushed or updated. ↩