feat(branding): Add new logo and update docs

[iambriccardo]

This PR adds a new temporary ETL logo for brand recognition. In addition, it adds our brand colors to MKDOCS and updates the README.

[imor]

This PR has made the following undesirable changes to the docs:

• The next and previous links at the bottom of the page are missing.
• The table of contents at the right side of a page is missing.
• The light/dark theme picker next to the search bar is missing.

The above match the wrappers https://fdw.dev/ docs and should be brought back.

Also a few pages were orphaned from the navigation which should be added back:

INFO    -  The following pages exist in the docs directory, but are not included in the "nav" configuration:
             - design/index.md
             - design/etl-crate-design.md
             - guides/index.md
             - guides/database-setup.md
             - reference/index.md
             - tutorials/getting-started.md

The reference section was also removed which we'd need. The docs are roughly modelled as explained here: https://diataxis.fr/

[imor]

Approved to unblock.