Add table-of-content navigation to the Lesson page

It would make sense to give more in-lesson navigation options for quick scrolling through the content. This also goes in hand with the addition of optional headers for content block (https://github.com/GDQuest/learn-gdscript/issues/19). We can also indicate quizzes on the TOC.

Additionally, it can help to address some concerns raised in https://github.com/GDQuest/learn-gdscript/issues/39 by adding a bookmark label that would scroll you to the last known position if you've visited the lesson before. It would also feature a "back-to-top" button.