add the ethical ads div

[NicolasMassart]

Pull request checklist

Use the following list to make sure your PR fits the Besu doc quality standard.

Before creating the pull request

Make sure that:

• all commits in this PR are signed off for the DCO.
• you read the contribution guidelines.
• you have tested your changes locally before submitting them to the community for review. Not possible as the ad is injected only when on RTD, so testing can be done only in PR build preview.

After creating your pull request and tests finished

Make sure that:

• you fixed all the issues raised by the tests, if any.
• you verified the rendering of your changes on ReadTheDocs.org PR preview
  and updated the testing link (see Testing).

Describe the change

Add the Ethical ads div code

Issue fixed

fixes #575

Impacted parts

For content changes

• Doc content
• Doc pages organisation

For tools changes

• CircleCI workflow
• Build and QA tools (lint, vale,…)
• MkDocs templates
• MkDocs configuration
• Python dependencies
• Node dependencies and JavaScript
• ReadTheDocs configuration
• GitHub integration

Testing

see https://hyperledger-besu--576.org.readthedocs.build/en/576/

Screenshots / recording

[bgravenorst]

Looks good to me.