Summary:

This PR:

• Adds compression to the sample.zip file.
• Combines all docs PDF build commands in pkgutils.py, and renames the localised versions.
• Adds docs HTML build command that outputs to dist_doc, and can be used for packaging. See Release novelWriter as RPM packages #907 and Add HTML docs build for packaging #2278.
• Fixes asset inclusion in image builds to include PDFs for all languages.
• Makes a few updates in the docs.

Related Issue(s):

Closes #2278

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All linting checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed