Add limit to table-of-contents depth.

[johnnymatthews]

The table-of-contents (TOC) currently has no depth limit, leading to very long TOCs like in /reference/rust-testing-interface/

Tasks

• Add a depth limit to the TOC which can be altered within this repo's hugo.yaml.

• Allow pages to ignore this limit by adding a per-page variable to a pages front-matter:

  ignore_toc_depth: true

  Or something like that.