Validator tool

[MarekSuchanek]

Design and develop tool for the Knowledge Model validation (now just for chapter files and directory structure, but with ability to extend easily in the future).

Design & development notes:

• use of JSON schema
• interactive for users (provide options, stop at 1st error -> recheck & continue, all at once, ...)
• test cases for CI
• clear but sufficient error/warning messages
• Python 3

Requirements:

• load KM (core & localizations)
• validate chapter files with JSON schema
• check naming conventions (number of chapter in filename, name of folder, ...)
• check references and other stuff that cannot be (or won't be) done via schema
• documentation