docs(exporter-chain): add exporter chain overview with default

[marcschaeferger]

This pull request introduces documentation for the new Exporter Chain feature in the OpenTelemetry Android RUM SDK. The main goal is to explain how the SDK handles asynchronous exporter initialization, buffering, and customization, ensuring reliable telemetry export even during startup or offline scenarios.

Documentation Additions:

• Added a new EXPORTER_CHAIN.md document detailing the exporter delegation chain, including in-memory and disk buffering, asynchronous initialization, and customization hooks.
• Updated README.md to reference the new Exporter Chain feature and documentation, highlighting its purpose and usage.

Motivation

Fills a gap in documentation around exporter-chain internals, configuration, and extension hooks that was request in Issue #1127
Addresses lack of centralized explanation of buffering layers, initialization order, and customization options.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.26%. Comparing base (d28a0e5) to head (ed0e29f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1243      +/-   ##
==========================================
- Coverage   64.29%   64.26%   -0.04%     
==========================================
  Files         142      142              
  Lines        3011     3011              
  Branches      296      296              
==========================================
- Hits         1936     1935       -1     
  Misses        998      998              
- Partials       77       78       +1     

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