ci: extract CI job that builds documentation site
Previously, the CI job pages
was building the documentation site, and,
since it publishes the results to https://asapo.pages.desy.de/
, the CI
job could only run on the develop
branch.
This commit extracts the CI job build-docs-site
that can run on other
branches. The resulting artifacts can be downloaded and served locally
by running npm run serve
.