add notebooks in `learn/core_notebooks` to the main examples gallery

[drbenvincent]

Following quick discussion here https://github.com/pymc-devs/pymc-examples/issues/83#issuecomment-1159736382 we'll add the notebooks of core features (listed here https://www.pymc.io/projects/docs/en/stable/learn/core_notebooks/index.html) to the main set of examples here https://www.pymc.io/projects/examples/en/latest/gallery.html.

Add as the first section in the gallery

Core notebook files are located here https://github.com/pymc-devs/pymc/tree/main/docs/source/learn/core_notebooks

[OriolAbril]

We'll need to upload the images locally (or generate custom thumbnails) and then use the grid directive with links of ref type. something like:

.. grid:: 1 2 3 4

  .. grid-item-card::
    :class-item: p-2 
    :img-top: thumbnail_overview.png
    :link: pymc:pymc_overview
    :link-type: ref

    PyMC Overview

  same for other notebooks

[drbenvincent]

And that goes at the top of gallery.rst in the pymc-examples repo?

[OriolAbril]

yep (didn't realize it was rst, updated the directives