Add authors field to frontmatter

[aaronamm]

We have many contributes who contribute to our projects.
We want to improve our system to search and list content easily by author and make the UI that can give credit to an author.
Therefore, we should add authors field to frontmatter at the head of each md/mdx files.

I think we should use authors (plural) because a document can have multiple authors.
The value should be an array, e.g.

authors: [ Mild, DMK ]

or

authors:
- Mild
- DMK

FYI, there are two ways to define array value in YAML, depending on personal preference.