Rename taxonomy kinds from taxonomy to term, taxonomyTerm to taxonomy

And we have taken great measures to limit potential site breakage:

* For `disableKinds` and `outputs` we try to map from old to new values if possible, if not we print an ERROR that can be toggled off if not relevant.
* The layout lookup is mostly compatible with more options for the new `term` kind.

That leaves:

* Where queries in site.Pages using taxonomy/taxonomyTerm Kind values as filter.
* Other places where these kind value are used in the templates (classes etc.)

Fixes #6911
Fixes #7395

docs/layouts/shortcodes/content-tree.html

@@ -0,0 +1,14 @@
+<div class="code">
+    <pre><code>
+        ├── blog
+        │   ├── _index.md [section]
+        │   ├── first-post.md [page]
+        │   └── second-post
+        │       ├── index.md [page bundle]
+        │       └── photo.jpg [page resource]
+        └── tags
+            ├── _index.md [taxonomy]
+            └── funny
+                └── _index.md [term]
+    </code></pre>
+</div>

docs/layouts/shortcodes/page-kinds.html

@@ -0,0 +1,7 @@
+| Kind | Description | Example |
+|----------------|--------------------------------------------------------------------|-------------------------------------------------------------------------------|
+| `home` | The landing page for the home page | `/index.html` |
+| `page` | The landing page for a given page | `my-post` page (`/posts/my-post/index.html`) |
+| `section` | The landing page of a given section | `posts` section (`/posts/index.html`) |
+| `taxonomy` | The landing page for a taxonomy | `tags` taxonomy (`/tags/index.html`) |
+| `term ` | The landing page for one taxonomy's term | term `awesome` in `tags` taxonomy (`/tags/awesome/index.html`) |