Changes in the general documentation layout

This PR make some general changes in the documentation web site

Update to the 1.0.8 version of ansys_sphix_theme
Adaptation of the PyAnsys logo for light and dark modes
Add a file with substitution text to be used around the doc for supported file formats per solver
Move the "Concepts" section into the "User guide" section
Add new extension in the conf file: "sphinx_exec_code" https://github.com/spacemanspiff2007/sphinx-exec-code

ansys / pydpf-core