add exercises

[brandelune]

https://www.reddit.com/r/emacs/comments/e9sb5q/new_introduction_to_programming_in_emacs_lisp/faz74w0/

I think an easy way to incorporate exercises into your text (because it essentially just means rearranging the text) would be to add a short one at the beginning of each section, then walk through the discovery process like you're already doing.
For example, at the beginning of the "Strings" section, you could add the following exercise: "Using describe-function, find out how the function message works. Use it to display a message about the value of the variable fill-column. Then expand this message to include octal and hexadecimal representations of the value." Then you could describe how to solve this, like you already do, and maybe introduce the possibility of following links in the help buffer (message -> format-message -> format) or how to search the index of the reference manual for 'formatting strings'.