docs: add usage page

CHANGELOG.md

@@ -6,6 +6,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 Also I copied this intro verbatim from [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 
 ## [Unreleased]
+### Added:
+* [Usage](https://thcon.app/usage) page in HTML docs
 
 ## [v0.17.0 - 2023-10-11]
 ### Added:

docs/content/usage.md

@@ -0,0 +1,54 @@
+---
+title: Usage
+layout: layouts/main.html
+eleventyNavigation:
+  parent: root
+  key: usage
+  title: usage
+---
+
+# Usage
+`thcon` usage happens in two phases. In the first, you configure your
+applications and write your `config.toml`. In the second, you use the `thcon`
+command line to make things dark/light and get back to what you were doing.
+
+## Configure Apps
+Consult the [app support table](https://thcon.app/apps/) and configure the
+applications you want to switch themes in using the included documentation.
+
+## Use CLI
+To switch all configured apps to dark mode, simply run:
+
+```text
+thcon dark
+```
+
+To go back to light mode:
+
+```text
+thcon light
+```
+
+To switch just some apps, list them after `dark` or `light`:
+
+```text
+# Lighten helix and iterm2, and macOS only.
+thcon light helix iterm2 macos
+```
+
+To view a list of apps supported on your platform, use `--help`:
+
+```text
+$ thcon dark --help
+Switches to apps to dark mode
+
+Usage:
+  thcon dark [app...] [flags]
+
+Apps:
+  alacritty, emacs, helix, iterm2, kitty, macos, neovim, terminal-app, vim
+
+Flags:
+  -h, --help            help for dark
+  -v, --verbose count   enable verbose logging (add multiple times for higher verbosity)
+```