Break the docs build on states that are acceptable for the current
NextJS-based docs engine but cause Docusaurus builds to fail:

- **Unresolved partial parameters:** Edit the `remark-includes` linter
  to ensure that all parameters declared within a partial (using `{{
  param }}` syntax) are either (a) assigned by the user or (b) given a
  default value. Otherwise, when we move to the new docs engine, builds
  will fail on unresolved parameters.

- **Add more config checks:** Throw an exception for duplicate redirects
  and redirects where the `source` points to an existing file. With the
  current logic, this check takes place with every page build. While
  this is not ideal, and leads to noisy error output, we should be
  migrating soon and will not need to deal with this for long.