docs: add meta descriptions to top-level pages (apollographql#12239)

Author: Meschreiber

docs/source/get-started.mdx

@@ -1,5 +1,7 @@
 ---
 title: Get started with Apollo Client
+subtitle: Set up Apollo Client and fetch GraphQL data with React
+description: Learn how to set up your React app, fetch GraphQL data, and integrate Apollo Client seamlessly into your UI with this step-by-step tutorial.
 ---
 
 Hello! 👋 This short tutorial gets you up and running with Apollo Client.

docs/source/index.mdx

@@ -1,5 +1,7 @@
 ---
 title: Introduction to Apollo Client
+subtitle: Learn how Apollo Client simplifies GraphQL data management to help build web apps
+description: Discover Apollo Client, a powerful state management library for JavaScript. Learn how it simplifies fetching, caching, and more with GraphQL.
 ---
 
 **Apollo Client** is a comprehensive state management library for JavaScript. It enables you to manage both local and remote data with GraphQL. Use it to fetch, cache, and modify application data, all while automatically updating your UI.

docs/source/why-apollo.mdx

@@ -1,6 +1,7 @@
 ---
 title: Why Apollo Client?
-description: Why choose Apollo Client to manage your data?
+subtitle: Discover why Apollo Client is the ideal choice for managing data with GraphQL
+description: Learn how Apollo Client simplifies data management with GraphQL, offers intelligent caching, and unifies remote and local data.
 ---
 
 Apollo Client is a state management library that simplifies managing remote and local data with GraphQL. Apollo Client's intelligent caching and declarative approach to data fetching can help you iterate faster while writing less code. Additionally, if you need custom functionality, you can create your dream client by building extensions on top of Apollo Client.