WEB: Add table of contents to the Ecosystem
Supersedes #61595

Changes to make the tables of contents in the website the same in all pages (we had customized the PDEP ones before).

Adding the ToC to Ecosystem. It renders like this:

![Screenshot at 2025-06-13 19-02-56](https://github.com/user-attachments/assets/a3471680-b9f9-46e6-b02d-0d379d75efec)


I can make it just one level if preferred. I think with one level looks nicer, but with two, it's uglier but more practical (and simpler since it's the same for all pages). But no strong preference from my side

Thanks @datapythonista 