docs: add Android troubleshooting for MP3 preview

[IbrarKhanCode]

PR Description

This PR adds Android troubleshooting documentation for an issue where MP3 previews fail to play due to Android blocking cleartext traffic to 127.0.0.1.

The documentation explains:

• The symptom (HttpDataSource$CleartextNotPermittedException while previewing MP3 files).
• The underlying cause (just_audio uses a localhost proxy for StreamAudioSource).
• The recommended solution using an Android Network Security Configuration to allow localhost cleartext traffic.
• An alternative workaround using usesCleartextTraffic, along with its security implications.

This issue was reproduced and the documented solution was verified during local Android development.

Related Issues

• Closes #

Checklist

• I have gone through the contributing guide.
• I have updated my branch and synced it with the project main branch before making this PR.
• I am using the latest Flutter stable branch.
• I have run the tests (flutter test) and all tests are passing.

Added/updated tests?

• Yes
• No, because this PR only updates documentation.

OS on which I have developed and tested the feature?

• Windows
• macOS
• Linux