Fix project archive download and simplify building the docs for a single project

[maximlt]

I think #290 broke downloading a project archive. This is fixed by

• making sure the archive file are in the doc/ folder
• using Sphinx download role to point to the archive, so Sphinx knows how to handle it and adds it automatically to the built site

This PR also streamlines building the docs for a single project, ignoring all other projects.

[github-actions]

Your changes were successfully integrated in the dev site, make sure to review
the pages of the projects you touched before merging this PR: https://holoviz-dev.github.io/examples/.
You can also download an archive of the site from the workflow summary page which comes in handy
when your dev site built was overriden by another PR (we have a single dev site!).

[github-actions]

Your changes were successfully integrated in the dev site, make sure to review
the pages of the projects you touched before merging this PR: https://holoviz-dev.github.io/examples/.
You can also download an archive of the site from the workflow summary page which comes in handy
when your dev site built was overriden by another PR (we have a single dev site!).

[github-actions]

Your changes were successfully integrated in the dev site, make sure to review
the pages of the projects you touched before merging this PR: https://holoviz-dev.github.io/examples/.
You can also download an archive of the site from the workflow summary page which comes in handy
when your dev site built was overriden by another PR (we have a single dev site!).