Add support for multiple authors in blog posts

[conradolandia]

This PR implements support for multiple authors in blog posts and OpenGraph image generation.

In a post, a single author still can be added as a string in the YAML header:

title: "This is an example post"
author: camgerlach
pub_date: 2023-12-19

Now, multiple authors should be added as an array:

title: "This is an example post"
author: 
  - camgerlach
  - ccordoba12
  - conradolandia
pub_date: 2023-12-19

As usual, the author must exist as a folder in /static/assets/authors, named after the author's GitHub username, containing an avatar image, and a metadata.json file with the following structure:

{
  "name": "C.A.M. Gerlach",
  "image": "C-Symbol-Logo-Full-V2.png"
}

The name field will be used in all instances as the author's name, and the image as the avatar.

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[ccordoba12]

Thanks @conradolandia! Great work, as usual.