11ty RSS feed integration

Add dependency

See https://www.mikestreety.co.uk/blog/creating-an-11ty-collection-from-json-api/ to understand.

We add the fetch 11ty plugin:

"devDependencies": {
    "@11ty/eleventy-fetch": "^4.0.0"
  }

We add the rss-parser plugin

"devDependencies": {
    "rss-parser": "^3.13.0"
  }

Then :

1. We call the RSS feed through the cache mechanism. This avoids calling the RSS feed everytime we hit enter while in development
2. We extract only the necessary attributes from the feed
3. We expose that as an array.

The file "blogPosts.js" in _data is exposed as a global blogPosts variable in the templates and layouts