[FEATURE] Support multiple categories in the same page

[andrecastro]

Idea

Add the possibility to render two categories in the same page.

Example

Defining multiple categories in the mkdocs.yaml:

# mkdocs.yaml
  - blogging:
      categories:
        - name: announcements
          dirs:
            - news/announcements
          size: 5
          theme:
            name: card
        - name: partner
          dirs:
            - tutorials/partner
          size: 5
          theme:
            name: card
        - name: cicd
          dirs:
            - tutorials/cicd
          size: 5
          theme:
            name: card

and them, adding two or more categories in the same page:

# Tutorials

## CICD

{{ blog_content cicd }}

## Partner

{{ blog_content partner }}

Currently behavior is rendering only the first content for both sections.