index: include mention of Limited Availability (#2724)

* index: include mention of Limited Availability

* Update gh-pages/src/index.md

Co-authored-by: Daniel D. Beck <daniel@ddbeck.com>

---------

Co-authored-by: Patrick Brosset <patrick.brosset@microsoft.com>
Co-authored-by: Patrick Brosset <patrickbrosset@gmail.com>
Co-authored-by: Daniel D. Beck <daniel@ddbeck.com>

Author: 3 people

gh-pages/src/index.md

@@ -12,6 +12,8 @@ Baseline features are available across popular browsers. Baseline has two stages
 - **Newly available**: The feature works across the latest devices and browser versions. The feature might not work in older devices or browsers. Indicated with a blue icon.
 - **Widely available**: The feature is well established and works across many devices and browser versions. It’s been available across browsers for at least 2½ years (30 months). Indicated with a green icon.
 
+Prior to being newly available, a feature has **Limited availability** when it's not yet available across all browsers.
+
 Baseline is calculated using the following core browser set:
 
 - Apple Safari (macOS and iOS)