Set a doc build variable for executorch version (#13351)

### Summary
Now in docs we can use `${executorch_version}` to represent executorch
version.

It's defined in docs/source/executorch_custom_versions.py
`EXECUTORCH_VERSION = "0.7.0"`

### Test plan

In
https://docs-preview.pytorch.org/pytorch/executorch/13351/getting-started.html#id1,
the version is correct:
```
implementation("org.pytorch:executorch-android:0.7.0")
```

Author: kirklandsign

docs/source/executorch_custom_versions.py

@@ -7,6 +7,9 @@
 """
 Sphinx extension to replace ${executorch_version:TAG} with version numbers.
 
+It also defines a special variable ${executorch_version} that is set to the value
+of `EXECUTORCH_VERSION` defined in this file.
+
 This custom extension pulls third-party version strings from files in the
 .ci/docker/ci_commit_pins directory, and uses them to expand specific strings in
 markdown files.
@@ -24,10 +27,13 @@
     "pytorch.txt",
 ]
 
+EXECUTORCH_VERSION = "0.7.0"
+
 variables: dict[str, str] = {}
 
 
-def read_version_files():
+def populate_version_variable():
+    variables["${executorch_version}"] = EXECUTORCH_VERSION
     cwd = os.getcwd()
     version_file_path = os.path.join(cwd, "..", ".ci", "docker", "ci_commit_pins")
 
@@ -38,7 +44,7 @@ def read_version_files():
             variables[var_name] = f.read().strip()
 
 
-read_version_files()
+populate_version_variable()
 
 
 def replace_variables(app, doctree, docname):

docs/source/getting-started.md

@@ -124,7 +124,7 @@ To add the library to your app, add the following dependency to gradle build rul
 ```
 # app/build.gradle.kts
 dependencies {
-  implementation("org.pytorch:executorch-android:0.6.0")
+  implementation("org.pytorch:executorch-android:${executorch_version}")
 }
 
 # See latest available versions in https://mvnrepository.com/artifact/org.pytorch/executorch-android

docs/source/using-executorch-android.md

@@ -28,13 +28,13 @@ The AAR library can be used for generic Android device with arm64-v8a or x86_64
 
 ExecuTorch is available on [Maven Central](https://mvnrepository.com/artifact/org.pytorch/executorch-android).
 
-Simply add the target [`org.pytorch:executorch-android:0.6.0-rc1`](https://repo.maven.apache.org/maven2/org/pytorch/executorch-android/0.6.0-rc1/) to your Android app dependency (build.gradle), and build your app.
+Simply add the target [`org.pytorch:executorch-android:${executorch_version}`](https://repo.maven.apache.org/maven2/org/pytorch/executorch-android/${executorch_version}/) to your Android app dependency (build.gradle), and build your app.
 
 For example:
 ```
 # app/build.gradle.kts
 dependencies {
-    implementation("org.pytorch:executorch-android:0.6.0-rc1")
+    implementation("org.pytorch:executorch-android:${executorch_version}")
 }
 ```
 
@@ -53,7 +53,8 @@ You can also directly specify an AAR file in the app. We upload pre-built AAR to
 
 | Version | AAR | SHASUMS |
 | ------- | --- | ------- |
-| [v0.6.0-rc1](https://github.com/pytorch/executorch/releases/tag/v0.6.0-rc1) | [executorch.aar](https://ossci-android.s3.amazonaws.com/executorch/release/v0.6.0-rc1/executorch.aar) | [executorch.aar.sha256sums](https://ossci-android.s3.amazonaws.com/executorch/release/v0.6.0-rc1/executorch.aar.sha256sums) |
+| [${executorch_version}](https://github.com/pytorch/executorch/releases/tag/${executorch_version}) | [executorch.aar](https://ossci-android.s3.amazonaws.com/executorch/release/${executorch_version}/executorch.aar) | [executorch.aar.sha256sums](https://ossci-android.s3.amazonaws.com/executorch/release/${executorch_version}/executorch.aar.sha256sums) |
+| [v0.6.0](https://github.com/pytorch/executorch/releases/tag/v0.6.0) | [executorch.aar](https://ossci-android.s3.amazonaws.com/executorch/release/v0.6.0/executorch.aar) | [executorch.aar.sha256sums](https://ossci-android.s3.amazonaws.com/executorch/release/v0.6.0/executorch.aar.sha256sums) |
 | [v0.5.0](https://github.com/pytorch/executorch/releases/tag/v0.5.0) | [executorch.aar](https://ossci-android.s3.amazonaws.com/executorch/release/v0.5.0-rc3/executorch.aar) | [executorch.aar.sha256sums](https://ossci-android.s3.amazonaws.com/executorch/release/v0.5.0-rc3/executorch.aar.sha256sums) |
 
 ### Snapshots from main branch
@@ -90,7 +91,7 @@ implementation("com.facebook.fbjni:fbjni:0.5.1")
 In your app working directory, such as executorch/examples/demo-apps/android/LlamaDemo,
 ```
 mkdir -p app/libs
-curl https://ossci-android.s3.amazonaws.com/executorch/release/v0.6.0-rc1/executorch.aar -o app/libs/executorch.aar
+curl https://ossci-android.s3.amazonaws.com/executorch/release/${executorch_version}/executorch.aar -o app/libs/executorch.aar
 ```
 
 And include it in gradle: