Unify docs.yml with corresponding file in keep-core

[michalinacienciala]

The docs.yml workflow config file has been modified in order to make it similar to the corresponding workflow in the keep-core. Most important changes include:

• Executing the workflow for PRs with all base branches, not only for those with master base branch
• Utilising dorny/paths-filter action (this will allow us to set the docs-html and docs-pdf jobs as required for a PR merge)
• Changing names of the workflow and jobs to make them unified with implementation of other workflows
• Fixing the name and id value for the step building PDF docs

[michalinacienciala]

@nkuba, before approving this PR, please set the docs-html and docs-pdf jobs as required for a PR merge.

[nkuba]

@michalinacienciala can we resolve conflict, please?