Add "Donate" Button to Website Sidebar

[namiroues]

We should add a "Donate" button to the website's sidebar to make it easier for donors to support MDAnalysis financially.

This will help promote sustainability and direct attention to funding opportunities (e.g., via NumFOCUS).

Tasks:

• Add a "Donate" button to the sidebar, below the privacy page.
• Link the button to the official donation page (e.g., https://numfocus.org/donate-to-mdanalysis).
• Remove the button from other pages to avoid duplication or clutter.
• Style it and use orange colour.
• Include a short tooltip or hover-text.

[namiroues]

closed by #464