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

[Copilot]

Pull request overview

Updates NTTT to handle modern RPF Markdown (> [!TASK]/> [!ACCORDION] … callouts) by adding two new processing modes — strip (removes non-translatable structural markers before Crowdin upload) and restore (reinserts them after download) — and integrates restore into the default tidy pipeline for non-English locales. Adds maintainer-facing documentation describing all transformations.

Changes:

• New nttt/markers.py, nttt/strip.py, nttt/restore.py modules implementing classification, stripping and round-trip restoration of modern and legacy markers, plus a new --mode CLI flag wired through arguments.py, constants.py, and __init__.py.
• tidyup.py now invokes restore_tree before the existing cleanup pass for non-English locales.
• Adds doc/transformations.md, README sections, fixture markdown files and unit tests covering strip/restore, marker classification, and the strip→restore roundtrip.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
nttt/markers.py	New marker classification (modern bare/labelled, legacy bare, empty blockquote).
nttt/strip.py	Implements strip_md / strip_tree removing structural markers outside fenced code.
nttt/restore.py	Implements restore_md / restore_tree and "looks already processed" guard.
nttt/tidyup.py	Calls restore_tree before per-file fix loop for non-English locales.
nttt/init.py	Dispatches CLI to strip/restore/tidy based on new --mode.
nttt/arguments.py	Adds -m/--mode CLI flag with tidy/strip/restore choices.
nttt/constants.py	Adds MODE argument key and Modes constants.
README.md	Documents new modes, pipx instructions, link to transformations.md.
doc/transformations.md	New maintainer-facing reference for NTTT's transformations.
unit_test/test_markers.py	Tests classify_line and helpers.
unit_test/test_strip.py	Tests strip_md behaviour incl. code-fence preservation.
unit_test/test_restore.py	Tests restore_md happy paths, warnings, and skip heuristics.
unit_test/test_strip_restore_roundtrip.py	Roundtrip tests using fixture markdown.
unit_test/test_cleanup_sections.py	Regression test that restore + fix_sections doesn't duplicate markers.
unit_test/test_arguments.py	Adds mode field to fake args and asserts defaults.
unit_test/data/markdown/*.md	New fixtures (accordions, nested markers, legacy quiz).

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