chore: use new about-us file

[HonkingGoose]

Changes:

• Drop old about-us.md file in the src directory

Context:

Follow up after the merge of renovatebot/renovate#15661.

I've left the nav config in the mkdocs.yml file the same, I didn't need to change anything, as the build picks up the new file in docs/usage automatically. 😄

I've ran make and confirmed that correct about-us is shown with the mkdocs serve site preview.

Closes #175.