Integrate Addon-Academy into new website

Currently, the [Addon Academy](https://freecad.github.io/Addon-Academy/) has its own repository, own URL via GitHub pages, own layout, and own build system via Jekyll.

The [new FreeCAD website](https://github.com/FreeCAD/Website) now integrates the [dev handbook](https://github.com/FreeCAD/DevelopersHandbook) content (previously using Jekyll too), in order to facilitate both content management and maintenance.

The new website even has a [sub-section for the Addon Academy](https://staging.freecad.org/dev/), visible on the top `dev` documentation section (in sixth place, with puzzle icon), currently leading to the GitHub pages website linked above.

What if the Addon Academy content is integrated into the new website, just like the dev handbook?

If yes, here are some open questions before achieving this work:

- Should the Addon Academy stay in its own repository? Both answers are possible, just need some different approaches to make it work.
- Should the current nested content structure be kept as-is or should we adopt a more flat structure? Both answers possible.
- Currently, almost not front matter is used. Titles, descriptions, weights, and other front matter fields will be added. Are some additional taxonomies (e.g. categories, tags, etc) wanted (dev handbook has no special taxonomy for now)?
- Any special features that would benefit the Addon Academy content or layout (i.e. that is not already available on the new website, I can list them if wanted), e.g. support for Latex, CSV, diagrams like D2, Mermaid, GraphViz, GoAT, Excalidraw, more JS stuff, etc?
- TBD

FYI @chennes @mnesarco

Start by comparing the Addon-Academy repository and its Jekyll structure with the FreeCAD/Website repository and its existing `/dev/` subsection. Resolve the repository, content structure, front matter, taxonomy, and special-feature questions before implementation; the work is done when the academy content is integrated into the new website and its existing navigation points to it.

Written by the indexing model from the issue text.