Change lesson steps titles

[zuevval]

See wiki article, point 7.

• remove lesson number and step number from titles of steps
• to each text step add a first line like "Lesson N, step M out of K" (automate it)
• change help messages (@zuevval)

It seems questionable whether this ⬆️ is the best possible solution, but I think it's OK. If there are better ideas, we may discuss them.

[winter-yuki]

Such a small problem should not cause any strange inconsistency. I think that it will be better not to solve it this way, maybe just replace Text with Theory for example.

[zuevval]

I’ve pondered a little and considered that you’re right. I will think about alternative word for title