Use more readable font-sizes for overviews and guides

[jelbourn]

No description provided.

[amcdnl]

LGTM

One thing I might suggest is decreasing the bottom margin more like Github has:

Below is a screenshot before/after with the following code applied:

.docs-markdown-h1,
.docs-markdown-h2,
.docs-markdown-h3,
.docs-markdown-h4,
.docs-markdown-h5 {
  margin-bottom: 16px;
}

Before

After

I might even suggest taking the margin bottom down a little more even.

[jelbourn]

I'll take a look at the margins in a follow-up PR.