doc: basic gallery of examples

[jorgepiloto]

This pull-request implements a gallery of examples in the documentation pages. Main changes include:

• Installing pandoc during the CI/CD from official CentOS7 repositories
• Running the doc-build job in the self-hosted runner
• Remove cell numbering, margins, and increase font size to 1rem
• Use Sphinx hooks to host the examples/ dir at the root directory of the project
• Enable script and notebook download links
• Option to render widgets in Jupyter sidecar

Gallery of examples with multiple sections and thumbnails

Download buttons get automatically included at the begging of every example

[jorgepiloto]

I just found that Python scripts can also be opened directly in Jupyter Lab. These scripts are rendered as notebooks instead of plain scripts. This are great news! It allows to get rid of some deps and reduces the complexity for newcomers. Performing another iteration on this...

[davegouger]

Only one very minor comment about hyphenating "step-by-step." in index.rst. I thought I had changed that recently but it looks like I may have missed it.