Move Android Java pages to platforms/android/java (#489)

Followup to #473, which made minimal changes -- it _only_ relocated the Android and Web pages without changing them. This PR moves the Android pages (which are Java specific) to a sublevel, like this:

- /docs/
  - platforms/
    - android/
      - **java/{quick-start, basics,...}**
      - kotlin/quick-start
      - dart or flutter ...
    - web/{quick-start, basics, ...}

Context: https://github.com/grpc/grpc.io/pull/473#issuecomment-714048028

.cspell.json

@@ -24,6 +24,7 @@
         "printf",
         "protoc",
         "pushback",
+        "relref",
         "RPC's",
         "RPCs",
         "runtimes",

content/docs/platforms/_index.md

@@ -13,8 +13,8 @@ Each gRPC [language][] / platform has links to the following pages and more:
 
 Select a platform to get started:
 
-- [Android Java](android/)
+- [Android]({{< relref "android">}})
 - iOS (coming soon)
-- [Web](web/)
+- [Web]({{< relref "web" >}})
 
 [language]: {{< ref "languages" >}}

content/docs/platforms/android/_index.md

@@ -1,10 +1,8 @@
 ---
-title: Android Java
-api_path: grpc-java/javadoc
+title: Android
 ---
 
-These language-specific pages are available:
+The following languages support client-side gRPC on Android:
 
-- [Quick start](quickstart/)
-- [Basics tutorial](basics/)
-- [API reference](api/)
+- [Java]({{<relref "java">}})
+- Kotlin (coming soon)

content/docs/platforms/android/java/_index.md

@@ -0,0 +1,10 @@
+---
+title: Android Java
+api_path: grpc-java/javadoc
+---
+
+These language-specific pages are available:
+
+- [Quick start](quickstart/)
+- [Basics tutorial](basics/)
+- [API reference](api/)

layouts/index.redirects

@@ -2,7 +2,7 @@
 
 # Languages -> platforms redirects
 
-/docs/languages/android/*  /docs/platforms/android/:splat
+/docs/languages/android/*  /docs/platforms/android/java/:splat
 /docs/languages/web/*  /docs/platforms/web/:splat
 
 # API reference docs
@@ -14,7 +14,7 @@
 #  /docs/languages/:lang/api  $api_path 301!                          # when $api_path starts with 'http'
 #  /docs/languages/:lang/api  https://grpc.github.io/$api_path 301!   # otherwise; 'LANG' in $api_path is replaced by :lang
 
-{{ $pages := (.Site.GetPage "/docs/platforms").Sections -}}
+{{ $pages := (.Site.GetPage "/docs/platforms/android").Sections -}}
 {{ $pages := $pages | union (.Site.GetPage "/docs/languages").Sections -}}
 {{ range $pages -}}
   {{ $from_path := printf "/%sapi" .File.Dir -}}