Add usage instructions to homepage

docs/index.qmd

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 The `embedio` shortcode extension allows you to incorporate different file types into Quarto HTML documents by using Quarto's shortcode mechanism.
+
+## Installation
+
+To install the `embedio` shortcode extension, follow these steps:
+
+1. Open your terminal.
+
+2. Execute the following command:
+
+```bash
+quarto add coatless-quarto/embedio
+```
+
+This command will download and install the shortcode extension under the `_extensions` subdirectory of your Quarto project. If you are using version control, ensure that you include this directory in your repository.
+
+## Usage
+
+We presently support the following shortcodes:
+
+| Shortcode  | Description                                                     |
+|------------|-----------------------------------------------------------------|
+| [`audio`](qembedio-embed-audio.qmd)    | Embeds an audio player with optional caption and download link. |
+| [`pdf`](qembedio-embed-pdf.qmd)      | Embeds a PDF file with customizable width and height.           |
+| [`revealjs`](qembedio-embed-revealjs.qmd) | Embeds a Reveal.js slide deck with optional height.             |
+
+You can use the shortcode immediately in a Quarto project that has the extension installed by typing into the document:
+
+```md
+{{< shortcode-name filename >}}
+```
+
+For example, we can embed RevealJS slides using:
+
+```md
+{{< revealjs "assets/my-slides.html" >}}
+```