Fix responsive column width in demo page for mobile browser

[KCarlile]

Background

When viewing the examples page on a mobile device, it's apparent that the 50/50 columns are not working for that narrow display. See the example screenshot below:

Steps to Reproduce

View https://demo.kcarlile.com/guitar-diagrams-js/ on a phone's browser or in a browser with the width set to a mobile size.

Acceptance Criteria

• The examples page is readable with responsive columns on mobile browsers without breaking the experience for fullsize browsers

Approach

• Using Bootstrap's classes, try changing <div class="col-6">...</div><div class="col-6">...</div> to something like <div class="col-md-6">...</div><div class="col-md-6">...</div>

Notes

• See Bootstrap's columns and breakpoint docs

[KCarlile]

Using <div class="col-md-6 mb-4">...</div> fixed this.

[KCarlile]

PR into develop: #61
PR into 10-productize-documentation: #62