Proposal: Add a quick summary of the prefixes to the docs

[RichardJActon]

I tend to forget what the different prefixes mean when I've not been using the library for a while and whenever I go to double check them it takes me a moment to parse through this section of the intro page: https://mllg.github.io/checkmate/articles/checkmate.html#function-overview

I think it would be helpful to have a quick at-a-glance reference for what the different prefixes mean, perhaps as a subsection of function overview?

Here's what I'd suggest - just slightly re-wording/restructuring what's already there to make it more succinct, separate it from the camelcase vs snakecase stuff and emphasize the important differences.

Checkmate Function Prefixes Summary

• assert*
  If prefixed with assert, an error is thrown if the corresponding check fails.
  Otherwise, the checked object is returned invisibly.
• test*
  The test family always return the check result as logical value.
• check*
  Functions starting with check return the error message as a string (or TRUE otherwise) and can be used if you need more control and, e.g., want to grep on the returned error message.
• expect*
  The expect family is intended to be used with the {testthat} package.
  All performed checks are logged into the testthat reporter.

Thanks for checkmate it's incredibly useful!