feature: Allow specifying page descriptions

[pawamoy]

Mentioned in https://github.com/pawamoy/mkdocs-llmstxt/pull/4/files#r2003344450:

plugins:
- llmstxt:
    markdown_description: Long description of my project
    sections:
      Usage documentation:
        - path: file1.md
          description: Desc
        - file2.md

[vladfaust]

IDEA: Provide page descriptions in front matter.

[pawamoy]

By the way it was implemented in #8, closing 🙂 If I didn't release it yet, I'll try to do that today.

[pawamoy]

@vladfaust that's a good idea, don't hesitate to open a new feature request 🙂