Integrate "categories and tags" post into "post lists"

I'm no longer displaying categories or tags in breadcrumbs or pills.

Categories and tags are just convenient ways to collect groups of posts
together so that you can create pages for individual categories or tags
using the post_list.html include.

Categories happen to appear in URLs as well.

_posts/2019-08-09-disclosure-boxes.md

@@ -1,3 +1,7 @@
+---
+tags: [Example Tag]
+---
+
 Disclosure Boxes: `<details>` and `<summary>`
 =============================================
 

_posts/2019-08-10-relative-links.md

@@ -1,3 +1,7 @@
+---
+tags: [Example Tag]
+---
+
 Relative Links
 ==============
 

_posts/2019-08-11-emoji.md

@@ -1,3 +1,7 @@
+---
+tags: [Example Tag]
+---
+
 Emoji
 =====
 

_posts/2019-11-25-header-and-footer-links.md

@@ -1,3 +1,7 @@
+---
+category: example-category
+---
+
 Header and Footer Links
 =======================
 

_posts/2019-12-22-drafts.md

@@ -1,3 +1,7 @@
+---
+category: example-category
+---
+
 Drafts
 ======
 

example-category/_posts/2019-12-22-post-lists.md

@@ -0,0 +1,106 @@
+Post Lists
+==========
+
+The `post_list.html` include renders lists of blog posts, with
+support for filtering by category or tag.
+
+## Listing all published posts
+
+To render a list of all the site's posts:
+
+```liquid
+{% raw %}{% include post_list.html %}{% endraw %}
+```
+
+## Listing draft posts
+
+To render a list of draft posts only, pass `drafts=true`:
+
+```liquid
+{% raw %}{% include post_list.html drafts=true %}{% endraw %}
+```
+
+## Listing posts in a category
+
+You can assign a category to a Jekyll post by putting the post's file in a `_posts/` dir within a subdir.
+For example if a post is in `bar/_posts/` rather than in the top-level `_posts/` dir,
+then the post will be in the `bar` category.
+Alternatively you can set a post's category by putting `category: bar` in the post's YAML frontmatter.
+
+To render only posts from a given category pass `category="some-category"` to `post_list.html`:
+
+```liquid
+{% raw %}{% include post_list.html category="some-category" %}{% endraw %}
+```
+
+You can use this to create a page for a category.
+One of this site's categories is `"example-category"`.
+To create a category page for `"example-category"` you can either
+create an `example-category.md` file or an `example-category/index.md` file
+and it will be rendered at `/example-category/`.
+
+The `/example-category/` URL is nice:
+if your [`permalink` setting](https://jekyllrb.com/docs/permalinks/) starts with `/:categories/`
+(all of Jekyll's [built-in permalink formats](https://jekyllrb.com/docs/permalinks/#built-in-formats) do)
+then the permalinks of posts in `example-category` will start with `/example-category/`,
+so they'll be sub-URLs of the category's page.
+
+For an example category page see this site's
+[`example-category/index.md`]({{ site.github.repository_url }}/blob/master/example-category/index.md) file,
+with renders this [page for the example category](../index.md).
+
+### Categories aren't hierarchical
+
+The fact that categories can appear in the filesystem and in URLs might imply
+that there's a hierarchical ordering of categories. For example you might think
+that a `foo/bar/` dir `gar/bar/` dir are two different categories:
+
+    foo/
+      bar/
+        _posts/
+          2019-12-09-first-post.md
+    gar/
+      bar/
+        _posts/
+          2019-12-09-second-post.md
+
+You might think that, as with directories in a filesystem, `foo/bar/` and
+`gar/bar/` are two different categories. But that isn't how it works in Jekyll:
+there's just a single `"bar"` category and both posts belong to it.
+There's no hierarchical ordering between the `"bar"`, `"foo"` and `"gar"` categories.
+The two directories `foo/bar/` and `gar/bar/` are just two different ways of putting posts in the `"bar"` category.
+`foo/bar/` also puts the post in the `"foo"` category, and `gar/bar/` also puts the post in the `"gar"` category.
+The first post belongs to two separate categories `"foo"` and `"bar"`, and the
+second post belongs to `"gar"` and `"bar"`:
+
+* Categories:
+  * foo:
+    * 2019-12-09-first-post.md
+  * bar:
+    * 2019-12-09-first-post.md
+    * 2019-12-09-second-post.md
+  * gar:
+    * 2019-12-09-second-post.md
+
+## Listing posts with a tag
+
+You can tag a post in Jekyll by adding a `tags` variable to its YAML front matter,
+for example: `tags: [hot, summer]`.
+
+To render the list of posts with a given tag pass `tag="some-tag"`  to `post_list.html`:
+
+```liquid
+{% raw %}{% include post_list.html tag="some-tag" %}{% endraw %}
+```
+
+You can use this to create a page for a tag.
+One of this site's categories is `"example-tag"`.
+To create a tag page for `"example-tag"` you can either
+create an `example-tag.md` file or an `example-tag/index.md` file
+and it will be rendered at `/example-tag/`.
+
+For an example category page see this site's
+[`example-tag.md`]({{ site.github.repository_url }}/blob/master/example-tag.md) file,
+with renders this [page for the example tag](../../example-tag.md).
+
+Unlike categories, [tags in Jekyll can't appear in URLs](https://jekyllrb.com/docs/posts/#categories-and-tags).

example-tag.md

@@ -3,4 +3,4 @@ Example Tag
 
 All posts tagged "Example Tag":
 
-{% include post_list.html tag="example-tag" %}
+{% include post_list.html tag="Example Tag" %}