Add checkdoc to CI test run

[fapdash]

To make sure that the exercise templates are following the Emacs Lisp coding and documentation conventions it would be nice to add checkdoc to the CI test run.
My suggestion would be to deactivate certain rules and fail on any checkdoc warnings.

Usage examples

• https://github.com/abo-abo/avy/blob/e92cb37457b43336b765630dbfbea8ba4be601fa/targets/checkdoc.el
• abo-abo/avy@15c7f9f
• https://github.com/flycheck/flycheck/blob/f8c679fff349850c80541a31de50009c3c15d4c9/maint/flycheck-checkdoc.el#L81

How to find files

• https://www.gnu.org/software/emacs/manual/html_node/elisp/Contents-of-Directories.html#index-directory_002dfiles_002drecursively

Deactivating rules

• https://emacs.stackexchange.com/questions/74143/is-there-a-way-to-deactivate-specific-rules-of-checkdoc
  • https://emacs.stackexchange.com/a/74148/11806
• checkdoc--argument-missing-flag, this is perfect!
  • https://github.com/emacs-mirror/emacs/blob/master/lisp/emacs-lisp/checkdoc.el#L342-L361