All exercises: Add checkboxes to the READMEs

[gs-101]

Because

The documentation refers to the lists in the READMEs as "self checks".

I think that having actual checkboxes for the README could show that Markdown has support for that, and it can make the tracking of progress better.

This PR

• Adds checkboxes to all the items under "Self check" headings
• Renames the heading "Self-check" in 01-margin-and-padding-1 to "Self Check" to make it more consistent with others.

Pull Request Requirements

• [X] I have thoroughly read and understand The Odin Project Contributing Guide
• [X] The title of this PR follows the location of change: brief description of change format, e.g. 01-flex-center: Update self check
• [X] The Because section summarizes the reason for this PR
• [X] The This PR section has a bullet point list describing the changes in this PR
• [ ] If this PR addresses an open issue, it is linked in the Issue section
• [ ] If applicable, I have ensured that the TOP solution files match the Desired Outcome image