Add licensing and attribution info to Brian's lessons

[Brian-Magnuson]

In short, this PR adds licensing and attribution information to the lessons by @Brian-Magnuson that did not already have such information. This mainly affects older lessons, when the formatting for the inclusion of this information was not well established at the time.

Later lessons have a footer section that looks roughly like this:

References

• Cppreference
• Other references

Graphics by (Author).

This work by (Author) is licensed under CC BY-SA 4.0.

Find a mistake? Open an issue on GitHub!

Old lessons have been updated to follow this format.