[antlir2][docs] document chef-solo and a few other features

Summary: We need better docs

Test Plan: debugging_without_docs

Reviewed By: epilatow

Differential Revision: D50885410

fbshipit-source-id: 799a38963a2822b363b09cda0075e0bf038c04b2

antlir/antlir2/docs/docs/features/_category_.json

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+{
+    "label": "Features",
+    "position": 20,
+    "collapsed": false
+}

antlir/antlir2/docs/docs/features/features.md

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+---
+sidebar_position: 1
+---
+
+# Features
+
+"Features" is the term that `antlir2` uses to describe user-provided
+instructions for how an image is to be built.
+
+A common misconception is that the order in which features are written in buck
+matter. They do not. Features are ordered by a
+[dependency graph](../internals/depgraph.md), so you don't have to concern
+yourself with the order in which you write your features.
+
+## Self-explanatory features
+
+:::note
+
+In the near future, this section will be replaced with an auto-generated section
+with the actual buck api docs.
+
+:::
+
+### `feature.install`
+
+Install a source file/directory (or output of a buck rule) into the image
+
+### `feature.ensure_dirs_exist`
+
+Create an entire directory chain owned by the same `user:group` and the same
+mode bits
+
+### `feature.ensure_subdirs_exist`
+
+Create some directories under an existing directory, where only the `user:group`
+and mode bits of the `subdirs_to_create` matter, as long as the parent
+directories exist.
+
+### `feature.genrule`
+
+Run an arbitrary command inside the image as any user that exists in the image.
+This cannot provide you any of the safety that `antlir2` normally provides, so
+should be used as a last resort.
+
+### `feature.remove`
+
+Delete a file/directory.
+
+### `feature.ensure_(file|dir)_symlink`
+
+Create a symlink to a file or directory.
+
+### `feature.rpms_install`
+
+Install RPMs by name, nevra or `.rpm` artifact.
+
+### `feature.rpms_upgrade`
+
+Upgrade RPMs to the newest allowed version.
+
+### `feature.rpms_remove_if_exists`
+
+Remove RPMs by name or nevra if they are installed.
+
+### `feature.(user|group)_add`
+
+Add a new user/group to `/etc/passwd` and friends.
+
+<InternalOnly>
+There are also some <a href="./fb">Meta-only internal features</a>
+</InternalOnly>

antlir/antlir2/docs/docs/recipes/_category_.json

@@ -1,5 +1,5 @@
 {
     "label": "Recipes",
-    "position": 20,
+    "position": 30,
     "collapsed": false
 }