Docs: Filtering Posts with categories, tags, or other variables (#6399)

Merge pull request 6399

docs/_docs/posts.md

@@ -155,6 +155,51 @@ you wish to access the currently-rendering page/posts's variables (the
 variables of the post/page that has the `for` loop in it), use the `page`
 variable instead.
 
+## Displaying post categories or tags
+
+Hey, that's pretty neat, but what about showing just some of your posts that are
+related to each other? For that you can use any of the [variables definable in
+Front Matter](https://jekyllrb.com/docs/frontmatter/). In the "typical post"
+section you can see how to define categories. Simply add the categories to your
+Front Matter as a [yaml
+list](https://en.wikipedia.org/wiki/YAML#Basic_components).
+
+Now that your posts have a category or multiple categories, you can make a page
+or a template displaying just the posts in those categories you specify. Here's
+a basic example of how to create a list of posts from a specific category.
+
+First, in the `_layouts` directory create a new file called `category.html` - in
+that file put (at least) the following:
+```html
+---
+layout: page
+---
+{% for post in site.categories[page.category] %}
+    <a href="{{ post.url | prepend: site.baseurl }}">
+      {{ post.title }}
+    </a>
+<hr>
+{% endfor %}
+```
+
+Next, in the root directory of your Jekyll install, create a new directory
+called `category` and then create a file for each category you want to list. For
+example, if you have a category `blog` then create a file in the new directory
+called `blog.html` with at least
+
+```text
+---
+layout: category
+title: Blog
+category: blog
+---
+```
+
+In this case, the listing pages will be accessible at `{baseurl}/category/blog.html`
+
+While this example is done with categories, you can easily extend your lists to
+filter by tags or any other variable created with extensions.
+
 ## Post excerpts
 
 Each post automatically takes the first block of text, from the beginning of