feat: Add implementations to forc-doc

[sdankel]

Description

Closes #5957

Also tweaked the CSS to make:

• the subheaders inside of implementations smaller than the impl header
• the colors a little less stark
• the impl toggle blocks open by default (same as crates.io)

Discovered two issues, relating to enums and the sidebar, which I'll solve separately: #6018

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[kayagokalp]

Left a single question to understand it better, great that we have this quickly!

[kayagokalp]

utACK 🚀

[JoshuaBatty]

Nice work Sophie! Thanks for whipping this up so quickly :)