Implement infrastructure for Nebula sub-site 🚀

[NPDebs]

Related to #765, #775, #841.

Deploy preview...

Screenshot:

Changes made:

• Adapted the prepare_website_data.py script to Nebula spreadsheet.
• Ran scripts to build basic infrastructure (of yaml files) and extract data from the planning sheet to populate them.
• Implemented "about page" design by Jilaga.
• Create conditional Nebula navbar design.

:+1::tada: First of all, thanks for taking the time to contribute! :tada::+1:

FOR CONTRIBUTOR

• [x] I have read the CONTRIBUTING.md document

PR Type:

• [ ] This PR adds a new blog post
• [x] This PR does something else (explain above)

FOR REVIEWERS

Thanks for taking the time to review! :heart:

Here are the list of things to make sure of:

• [ ] The website builds (a check will fail if not)
• [ ] All images have been added within the Pull Request and they have Alt text
• [ ] If there are paragraphs or text, the key messages are highlighted
• [ ] All internal links (within OLS website) use the {% link path_to_file.md %} format
• [ ] The preview corresponds to the changes described in the Pull Request
• [ ] The code is tidy and passes the linting tests