Update the local build instructions

Use case description

From https://github.com/androidx/media/blob/release/README.md#locally

You should now see the AndroidX Media modules appear as part of your project. You can depend on them from build.gradle.kts as you would on any other local module, for example:

implementation(project(":media3-lib-exoplayer"))

Proposed solution

Demonstrate changing via substitution rules in build.gradle.kts

val media3Checkout = project.properties["media3Checkout"]?.toString() ?: ""

if (media3Checkout.isNotBlank()) {
    allprojects {
        configurations.all {
            resolutionStrategy {
                dependencySubstitution {
                    substitute(module("androidx.media3:media3-common")).using(project(":media3-lib-common"))
                    substitute(module("androidx.media3:media3-datasource-okhttp")).using(project(":media3-lib-datasource-okhttp"))
                    substitute(module("androidx.media3:media3-exoplayer")).using(project(":media3-lib-exoplayer"))
                    substitute(module("androidx.media3:media3-exoplayer-dash")).using(project(":media3-lib-exoplayer-dash"))
                    substitute(module("androidx.media3:media3-exoplayer-hls")).using(project(":media3-lib-exoplayer-hls"))
                    substitute(module("androidx.media3:media3-exoplayer-rtsp")).using(project(":media3-lib-exoplayer-rtsp"))
                    substitute(module("androidx.media3:media3-exoplayer-workmanager")).using(
                        project(
                            ":media3-lib-exoplayer-workmanager"
                        )
                    )
                    substitute(module("androidx.media3:media3-session")).using(project(":media3-lib-session"))
                    substitute(module("androidx.media3:media3-test-utils")).using(project(":media3-test-utils"))
                    substitute(module("androidx.media3:media3-test-utils-robolectric")).using(
                        project(":media3-test-utils-robolectric")
                    )
                    substitute(module("androidx.media3:media3-ui")).using(project(":media3-lib-ui"))
                }
            }
        }
    }
}

This could be made simpler replacing by a pattern.

Alternatives considered

Keep the existing advice, however it can't be copy and pasted and requires editing lots of files.