[BIZ-2078] Fix broken links, update stale ones. Small wording improvements.

This MR resolves #37 (closed), #36 (closed), #34 (closed), #28 (closed) and #18 (closed). It does so by fixing several broken links, updating stale ones, and fixing wording/typographical issues.

JIRA tickets: https://tasks.opencraft.com/browse/BIZ-2078

Testing instructions:

1. Clone repository, rename the repo directory to course and from outside of it, tar -cvzf course.tar.gz course.
2. Upload the created course.tar.gz file as a course import to the onboarding-dev course. Run through the issues mentioned in the description and verify that all are resolved (note: the URLS in those issues will point to the live course instead of the dev one, so you will have to adjust them to verify)

Author notes and concerns:

I've removed the link to the Open edX architecture course in favor of a couple of overviews of the components we have available. Looking over the course I found a lot of things that were half-thrown together and incomplete, so I figured it would be better to just remove it and put something else in that would give the most important info, which happened to be a blog post and a video.