Add `raw-loader` dependency for building `docs/site`

\+ Remove `npm install raw-loader` step from CI job `pages`

Without the `raw-loader` package, building `docs/site` by executing the following command was failing:

```sh
$ cd docs/site && npm install
$ yarn build
...
[ERROR] Docusaurus server-side rendering could not render static page with path /docs/21.06.0/.
...
Error: Cannot find module './versioned_examples/version-21.06.0/start_asapo_socket.sh'
...
[WARNING] {"moduleIdentifier":"/home/marc-olivier/Documents/desy/dev/gitlab.desy.de/asapo/asapo/docs/site|sync|/(\\.sh%7C\\.py%7C\\.cpp%7C\\.c%7C\\.txt%7CMakefile)$/","moduleName":"././ sync (\\.sh%7C\\.py%7C\\.cpp%7C\\.c%7C\\.txt%7CMakefile)$","loc":"./CMakeLists.txt","message":"Module not found: Error: Can't resolve 'raw-loader' in '/home/marc-olivier/Documents/desy/dev/gitlab.desy.de/asapo/asapo/docs/site'","compilerPath":"client"}
...
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
```