tidying the homepage a bit - index for lessons

[sneakers-the-rat]

I was a bit confused by the current homepage, since it wasn't clear to me where the lessons were!

Judging from the name of the repo/url, i am assuming that we'll want to have a number of different lessons in here over time? It made sense to me to have those be in the top bar with links out to other pages in the dropdown, so then it becomes clear that i am "in" one lesson or another by the top nav bar, and then the left bar is what part of the lesson i'm a part of. I would think that this whole site is "lessons" so it wasn't immediately obvious to me that i needed to click 'lessons' to go to the clean code lesson.

I also tidied up the rest of the index page, moved the contributors into a badge like the others, and added a card for the lesson like we have on the package guide. I think the header links are a little goofy just hovering in the center, so i aligned them to the left too.

The thing that i wasn't sure about here is whether the "functions conditionals and checks" is a different lesson than "clean, expressive, code" or whether it was a subsection of it? The folder structure says that it's a subsection, but ig the hierarchy there was a little ambiguous to me so lmk if that's supposed to be considered a separate lesson.

[lwasser]

@all-contributors please add @sneakers-the-rat for code, review

[allcontributors[bot]]

@lwasser

I've put up a pull request to add @sneakers-the-rat! :tada: