Add example of primitive string substitution

[straight-shoota]

Follow-up on https://github.com/crystal-lang/crystal-book/pull/484#discussion_r600847339

/cc @oprypin

[bcardiff]

I don't think not_nil! should be shown/encouraged in the basic tutorial.

I prefer if/nop or if/raise.

In the methods/classes tutorial we could define a #trust_me_is_not_nil and show the reader there is nothing special about not_nil and then mention its existence in the std-lib.

But presenting it as in this PR sounds like we are encouraging it's use a lot.

[straight-shoota]

I'm not comfortable with promoting .not_nil! either.

But what else could it be? We need some way to make String#index usable. Maybe there should just be String#index! 😆 Conditionals are only introduced in the next lesson...

I suppose we might be able to do something like index || abort "Search string not found". Boolean algebra is also introduced in the next lesson, but maybe it's okay to just say "use this, it'll be explained later".

Any other ideas?

[oprypin]

What's wrong with if !search_index.nil?

[straight-shoota]

if gets introduced in the next lesson.

[bcardiff]

If the tutorials are to be read in strict order, then the things introduced in this PR should not go intro Basic.

Maybe a followup as Text manipulation technique section is a better fit than a basic tutorial. This way you can assume that basic knowledge is covered. I think that from "Indexing Substrings" until the end could be moved out of basic if we do this.

Otherwise I would use if search_index with a reference to the next section. Any other alternative is harder to understand for a newcomer.

My vote is to keep the basics short and have another section to describe more use cases were we can assume all basic tutorial are covered.