* Use altairplot Sphinx directive of sphinxext_altair package

* Use sphinxext_altair package from pypi

* Add note about sphinxext-altair package in release 5

* Add validation of Vega-Lite schema itself

* Install check-jsonschema

* MAINT: bump version to 5.0.0

* MAINT: bump version to 5.0.0

* deprecate .ref() over removing

* update changes.rst

* Update doc/releases/changes.rst

Co-authored-by: Stefan Binder <binder_stefan@outlook.com>

---------

Co-authored-by: Stefan Binder <binder_stefan@outlook.com>

Fix #3055.
This PR changes the description so it is in line with the example connected to it. 
I think the specification is good as is, so the emphasis is on the `alt.Y2` and not on the `ci0` and `ci1`.

* Make start page wider, example gallery on start page bigger, and show 7 examples per row

* Remove raw-html role as unclear what it does and can't see an effect when removing it

* Style first paragraph as a lead, similar to Vega-Lite docs

* Add cards to start page. Move reference to Vega-Lite docs to Overview

* Fix link to API section by renaming it to lowercase. Add back raw-html role as it is needed. It registers raw-html as being the raw role with html format for the rest of the documentation

* Make 'Vega-Altair' bold. Improve card grid layout for smaller screens

* Change api.rst file name also to lowercase in schema generation script

* Increase space around lead. From code suggestion

* Remove unused css

* Extend gallery to whole screen. Make text narrower

* Make gallery fill full width of header. Replace and reorder some gallery images so it looks better in this configuration. Replaced the heatmp with text as it looks too 'zomed in' with the text compared to the others

)

* Change titleparams to title in examples

* Sync scatter plot table examples

)

* Fix for when subcharts do not have layer_props

* Test the to_json and from_json methods using all examples in the gallery