Update NTTT to handle modern markdown

[jamdelion]

I've been testing out the GH actions in this test repo: https://github.com/raspberrypilearning/test-markdown-alien

Main changes in this PR:

• add transformations.md to document in plain English what exactly the NTTT script does
• Make changes to the script to be able to parse/tidy modern RPF markdown
• Add in restore/strip functionality to replace the existing hide-strings.yml action that is implemented in some projects. This is to hide non-translatable elements like --- task --- and ![TASK].

Next steps:

• Check behaviour with translation team
• Update project (default project) workflows to use updated NTTT commands: Update all workflows to match test repo default-project-template#12