improve SEO: titles and descriptions for NFT track lessons

*We need to homogonise lesson titles and descriptions (including in the lesson body .mdx texts)*

The titles in the front matter in the .mdx files don't match the titles in the 'Tracks' or 'Get Started' landing pages. It would be nice to use the same titles in all places including in the lessons.

Also, we need to take into consideration of the descriptions of the lessons in 'Tracks' and also in the lessons themselves. Quite often in these two places, we refer to e.g. lesson 1, or lesson 4 when these 'numbers' aren't represented in the actual lesson titles.