Add Table of Contents to README

[K-letir-rgb]

Description of the changes being introduced by the pull request:

Adds a Table of Contents to the README for easier navigation, as suggested in #2654.

This is a small, focused change per @jku's feedback on the earlier PR (#2655) that README improvements should be split into minimal, single-purpose PRs rather than one large combined change. Happy to follow up with separate small PRs for other items from the list if this looks good.

This is a documentation-only change; no tests apply.

Addresses #2654 (partial - table of contents only)

[jku]

My initial feedback is the same as in #2655:

GitHub already gives us an automatic Table of Contents (just click the hamburger menu) on https://github.com/theupdateframework/python-tuf ... With that in mind I'm not sure we want to add TOC that we have to manually maintain.

[K-letir-rgb]

That's a fair point, I wasn't aware of the built-in GitHub TOC feature. Given that, would it still be useful to have an inline TOC for people viewing the raw README (e.g. via cat README.md, or a text editor, or in contexts where GitHub's rendering isn't available), or would you rather I close this and look at a different item from the list instead?