Doc more changes sphinx-panels -> sphinx-design

[chrisjsewell]

In addition to: https://github.com/executablebooks/sphinx-design/blob/main/docs/get_started.md#migrating-from-sphinx-panels

link-button -> button-ref/button-link

and classes to options:

• stretched-link -> :click-parent:
• btn-block -> :expand:

[chrisjsewell]

.. panels:: -> .. grid:: 1 2 2 2 with about :gutter: 2

then --- -> .. grid-item-card:: and move first line to be first argument

[chrisjsewell]

:fa:name,class -> :fa:name;class