Add author filtering to `/blog`

[natestemen]

Description

Currently, blog posts only support a single author in the author field of the YAML header. To enable filtering by author, we need to:

1. Modify the author field in the YAML header to support a list of authors instead of a single string.
2. Implement filtering logic in JavaScript to allow users to filter blog posts by post.author.

Notes

• How should the author filtering UI will be presented?
• Best to maintain backward compatibility with existing blog posts that have a single string author.