feat: add comprehensive training documentation for _Executable class

[MonaaEid]

Description
This pull request introduces comprehensive training documentation for the _Executable class, aimed at SDK developers.

• The documentation explains the core execution flow, retry logic, error handling, logging, and debugging strategies for transactions and queries in the Hedera SDK.

• This addition is intended to help both new and existing contributors understand and troubleshoot the SDK's execution engine more effectively.

Documentation Additions:

• Added a detailed training guide for the _Executable class in docs/sdk_developers/training/executable.md, covering execution flow, retry logic with exponential backoff, error handling, logging, debugging tips, and practical usage examples.
• Updated CHANGELOG.md to include the addition of the comprehensive _Executable class training documentation.Description:

Related issue(s):

Fixes #909

Checklist

• Updated CHANGELOG.md
• Documented

[Copilot]

Pull request overview

This pull request introduces comprehensive training documentation for the _Executable class, which serves as the core execution engine for the Hedera SDK. The documentation is designed to help SDK developers understand the internal workings of transaction and query execution, including retry logic, error handling, and debugging strategies.

• Added a detailed 346-line training guide covering execution flow, retry mechanisms with exponential backoff, error handling patterns, logging strategies, and debugging workflows
• Included Mermaid diagrams, code examples, and practical debugging checklists
• Updated CHANGELOG.md to document this new training resource

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 20 comments.

File	Description
docs/sdk_developers/training/executable.md	Comprehensive training guide for the _Executable class with sections on execution flow, retry logic, error handling, logging, and debugging techniques
CHANGELOG.md	Added changelog entry documenting the new training documentation

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[MonaaEid]

@exploreriii please review when you have time.

[exploreriii]

Request review @manishdait @tech0priyanshu if possible

[manishdait]

Hi @MonaaEid, Some changes needed for the proper formatting of the code blocks.

[tech0priyanshu]

@MonaaEid Everything looks good. You already have a comprehensive review from @manishdait and Copilot.
Just a suggestion you can convert this to a draft PR until another review is needed.

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[exploreriii]

Thank you @MonaaEid @tech0priyanshu and @manishdait !
Excellent PR and great contribution