Nested panels

[awvwgk]

Is your feature request related to a problem? Please describe.

I'm trying to migrate a website to sphinx and sphinx-panels has been incredible helpful so far to create the bootstrap layouts we have previously written directly in HTML/CSS. Now I'm trying to nest columns inside a grid layout as described in the bootstrap documentation here, but without success to far. Nested panels are not described in the sphinx-panels documentation, therefore this seems more like a feature request than an actual bug report to me.

The following myst markdown

````{panels}
:column: col-lg-12

```{panels}
First
---
Second

---

Third
---
Forth

Currently yields separate and ill-formatted columns:

Describe the solution you'd like

Eventually, I want to produce a layout like this, using two columns and cards with headers inside the columns for the individual entries:

Using nested panels seems like the easiest way to make this work.

Describe alternatives you've considered

Trying to build the layout myself using the .. div:: directives, but this would be the last resort.

Additional context

Conda environment used (sorry for the pip/conda-forge mix):

``` conda list _libgcc_mutex 0.1 conda_forge conda-forge _openmp_mutex 4.5 1_gnu conda-forge ablog 0.10.13 pypi_0 pypi alabaster 0.7.12 py_0 conda-forge attrs 20.3.0 pypi_0 pypi babel 2.9.0 pyhd3deb0d_0 conda-forge beautifulsoup4 4.9.3 pyhb0f4dca_0 conda-forge brotlipy 0.7.0 py39h3811e60_1001 conda-forge ca-certificates 2020.12.5 ha878542_0 conda-forge certifi 2020.12.5 py39hf3d152e_1 conda-forge cffi 1.14.5 py39he32792d_0 conda-forge chardet 4.0.0 py39hf3d152e_1 conda-forge click 7.1.2 pyh9f0ad1d_0 conda-forge colorama 0.4.4 pyh9f0ad1d_0 conda-forge cryptography 3.4.4 py39h95dcef6_0 conda-forge docutils 0.16 py39hf3d152e_3 conda-forge feedgen 0.9.0 pypi_0 pypi idna 2.10 pyh9f0ad1d_0 conda-forge imagesize 1.2.0 py_0 conda-forge invoke 1.5.0 pypi_0 pypi jinja2 2.11.3 pyh44b312d_0 conda-forge latexcodec 2.0.1 pypi_0 pypi ld_impl_linux-64 2.35.1 hea4e1c9_2 conda-forge libffi 3.3 h58526e2_2 conda-forge libgcc-ng 9.3.0 h2828fa1_18 conda-forge libgomp 9.3.0 h2828fa1_18 conda-forge libstdcxx-ng 9.3.0 h6de172a_18 conda-forge lxml 4.6.2 pypi_0 pypi markdown-it-py 0.6.2 pypi_0 pypi markupsafe 1.1.1 py39h3811e60_3 conda-forge mdit-py-plugins 0.2.5 pypi_0 pypi myst-parser 0.13.5 pypi_0 pypi ncurses 6.2 h58526e2_4 conda-forge openssl 1.1.1k h7f98852_0 conda-forge packaging 20.9 pyh44b312d_0 conda-forge pip 21.0.1 pyhd8ed1ab_0 conda-forge pybtex 0.24.0 pypi_0 pypi pybtex-docutils 1.0.0 pypi_0 pypi pycparser 2.20 pyh9f0ad1d_2 conda-forge pydata-sphinx-theme 0.6.3 pypi_0 pypi pygments 2.8.0 pyhd8ed1ab_0 conda-forge pyopenssl 20.0.1 pyhd8ed1ab_0 conda-forge pyparsing 2.4.7 pyh9f0ad1d_0 conda-forge pysocks 1.7.1 py39hf3d152e_3 conda-forge python 3.9.2 hffdb5ce_0_cpython conda-forge python-dateutil 2.8.1 pypi_0 pypi python_abi 3.9 1_cp39 conda-forge pytz 2021.1 pyhd8ed1ab_0 conda-forge pyyaml 5.4.1 pypi_0 pypi readline 8.0 he28a2e2_2 conda-forge requests 2.25.1 pyhd3deb0d_0 conda-forge setuptools 49.6.0 py39hf3d152e_3 conda-forge six 1.15.0 pyh9f0ad1d_0 conda-forge snowballstemmer 2.1.0 pyhd8ed1ab_0 conda-forge soupsieve 2.2 pypi_0 pypi sphinx 3.5.1 pyhd8ed1ab_0 conda-forge sphinx-book-theme 0.1.0 pypi_0 pypi sphinx-copybutton 0.3.1 pypi_0 pypi sphinx-panels 0.5.2 pypi_0 pypi sphinx-togglebutton 0.2.3 pypi_0 pypi sphinx_rtd_theme 0.5.1 pyhd3deb0d_0 conda-forge sphinxcontrib-applehelp 1.0.2 py_0 conda-forge sphinxcontrib-bibtex 2.2.0 pypi_0 pypi sphinxcontrib-devhelp 1.0.2 py_0 conda-forge sphinxcontrib-htmlhelp 1.0.3 py_0 conda-forge sphinxcontrib-jsmath 1.0.1 py_0 conda-forge sphinxcontrib-qthelp 1.0.3 py_0 conda-forge sphinxcontrib-serializinghtml 1.1.4 py_0 conda-forge sqlite 3.34.0 h74cdb3f_0 conda-forge tk 8.6.10 hed695b0_1 conda-forge tzdata 2021a he74cb21_0 conda-forge urllib3 1.26.3 pyhd8ed1ab_0 conda-forge watchdog 2.0.2 pypi_0 pypi wheel 0.36.2 pyhd3deb0d_0 conda-forge xz 5.2.5 h516909a_1 conda-forge yaml 0.2.5 h516909a_0 conda-forge zlib 1.2.11 h516909a_1010 conda-forge ```

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[chrisjsewell]

This will essentially be closed by https://github.com/executablebooks/sphinx-design/issues/6