Adjusts Southeast Asian countries list

[ajtran303]

This PR will close #271

This enhances the layout of the countries listed in the definition for Southeast Asian by using two columns.

Screenshots before and after:

First, I changed the markdown to have better ease of reading:

• Adds h3 headings to Mainland / Maritime sub-regions
• Each country is now a list item element

Then, I added new CSS rules.

& h3 ~ ul {
  border-bottom: 0.1rem solid lightgrey;
  padding-bottom: 1rem;
}

Future definitions can use this new layout by following this Markdown example:

### Sandwich Condiments

- Mayonaise
- Mustard

### Pasta Sauces

- Marinara
- Pesto

Or this HTML example:

<h3>Sandwich Condiments</h3>

<ul>
  <li>Mayonaise</li>
  <li>Mustard</li>
</ul>

<h3>Pasta Sauces</h3>

<ul>
  <li>Marinara</li>
  <li>Pesto</li>
<ul>

[ajtran303]

Reviewers be aware: #321 has another approach to solving this as well!

[ajtran303]

Just an update on this: I'm finishing up school and focusing on graduating but after this I will come back and then work in this and get more involved in the API side too :)

[tatianamac]

Thanks for the update @ajtran303. Best of luck with school! We'll be here when you're ready.

[tatianamac]

@ajtran303 Is this good to merge in from your perspective?

[ajtran303]

@ajtran303 Is this good to merge in from your perspective?

@tatianamac ~Hi! This is good to merge in :)~

This may not be necessary to merge in, as there have been some styling updates that affect the need for this code.