use softprops/action-gh-release to attach artifact to release

[benoitmartin88]

fixes #686

List of things to check before making a PR

Before merging your code, please check the following:

• you have added a line describing your changes to the Changelog;
• you have added unit tests for any new or improved feature;
• in case you updated dependencies, you have checked pdi/docs/CheckList.md;
• in case of a change in pdi.h, this same change must be reflected in no-pdi/include/pdi.h;
• you have checked your code format:
  • you have checked that you respect all conventions specified in CONTRIBUTING.md;
  • you have checked that the indentation and formatting conforms to the .clang-format;
  • you have documented with doxygen any new or changed function / class;
• you have correctly updated the copyright headers:
  • your institution is in the copyright header of every file you (substantially) modified;
  • you have checked that the end-year of the copyright there is the current one;
• you have updated the AUTHORS file:
  • you have added yourself to the AUTHORS file;
  • if this is a new contribution, you have added it to the AUTHORS file;
• you have added everything to the user documentation:
  • any new CMake configuration option;
  • any change in the yaml config;
  • any change to the public or plugin API;
  • any other new or changed user-facing feature;
  • any change to the dependencies;
• you have correctly linked your MR to one or more issues:
  • your MR solves an identified issue;
  • your commit contain the Fix #issue keyword to autoclose the issue when merged.

[benoitmartin88]

github upload-artifact creates an artifact (that expires after 90 days) but does not attach it to a release.
By using softprops/action-gh-release, the produced pages.zip is attached to the release (does not expire), making it then available for download to build the pdi.dev website.