Clearly explain how to enable smart flank for Android in documentation

[lcdsmao]

Author the user story for this feature

As an Android developer, I want the documentation to clearly explain how to enable smart flank for Android.

Describe the solution you'd like
After exploring the source code, I found the config need to fulfill two requirements:

• Set a valid value to smart-flank-gcs-path
• Set either the legacy-junit-result or full-junit-result to be true

It would be better if the document could state these two points.😀

[kozaxinan]

I am also struggling to enable smart sharding feature. According to code https://github.com/Flank/flank/blob/master/test_runner/src/main/kotlin/ftl/reports/util/ReportManager.kt#L133. Like original post suggest, legacy-junit-result or full-junit-result to be true. It is introduced in #798. And full-junit-result is not supported by flank doctor yml check.