Create Related Content Block

### Description

Create a block that can be used with full-site editing to display a list of related posts. Out of the box, it should display 3 posts in a horizontal row.

### Use Case

> As an editor, I want related posts to appear below the post content on an article page.

### Acceptance Criteria

- This issue builds on the plugin templates structure defined in #51
- The block should be able to be placed below post content using full site editing and should display a list of 3 related posts horizontally in a row, containing the featured image and post title, both of which should link to the post.
- Add to the single post template created in [create-wordpress-theme](https://github.com/alleyinteractive/create-wordpress-theme/issues/9)

### Technical Details

- This block should assume that [Elasticsearch Extensions](https://github.com/alleyinteractive/elasticsearch-extensions) is installed, active, indexed, and configured.
- Use a [More Like This query in Elasticsearch](https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-mlt-query.html) to populate the list of posts.
- Ensure that the current post is not in the list of related posts (it shouldn't be, but confirm).
- Match based on post title, post content, categories, and tags.
- The block should be placed in `plugin-templates/blocks/theme-related-content` and the category for the block should be `theme`.