Change `Update Gemfile` in Quick-Start Guide to a bigger heading size.

[leochoo]

Summary

Hi, I'm new to Jekyll. I found this fantastic Jekyll theme and I love the simplicity of it!
But I would like to make one suggestion for the documentation.

• Change Update Gemfile to a bigger heading size.
  On Quick-Start Guide page, Update Gemfile is in a smaller size heading, which makes it look like it is a sub-step of Migrating to Gem Version. However, changing _config.yml and Gemfile must be done when switching to a new theme. So I would suggest to change the heading to a bigger size so that people don't miss it.

Motivation

Drawbacks

[iBug]

Looks like a strange setting with H4 of font-size: 0.75em. Might be better to cancel this altogether.

[mmistakes]

This is the hierarchy for headings... example below.

https://mmistakes.github.io/minimal-mistakes/markup/markup-html-tags-and-formatting/

h4 could potentially be changed to 0.9em, h5 to 0.8em, and h6 to 0.75em for a smoother transition of sizes. But there's nothing wrong with the documentation file. "Update Gemfile" is a subheading under "MIgrating to a Gem Version" which is why it was given a h4 designation.

[iBug]

@mmistakes May I ask to change the default size so at least H4 is no smaller than P? This is really an issue as discussed here: https://ux.stackexchange.com/q/126039/116810

[mmistakes]

Headline sizes have been changed so type hierarchy is clear. h1 > h2 > h3 > h4 > h5 > h6 > p

/* headline scale */
$h-size-1: 1.563em !default; // ~25.008px
$h-size-2: 1.25em !default; // ~20px
$h-size-3: 1.125em !default; // ~18px
$h-size-4: 1.0625em !default; // ~17px
$h-size-5: 1.03125em !default; // ~16.5px
$h-size-6: 1em !default; // ~16px