docs: add query training guide and custom query example

[Om7035]

Description:
Adds detailed documentation for the Query class to the SDK developer training guide and includes a code example for creating custom queries.

• Add docs/sdk_developers/training/query.md explaining Query architecture, execution flow, payment handling, and retry logic.
• Add examples/query/custom_query_example.py demonstrating how to subclass Query to create a custom implementation.

Related issue(s):

Fixes #910

Notes for reviewer:
This PR addresses the need for better internal documentation regarding the Query class. The new guide explains how Query inherits from _Executable and handles common tasks like node selection and retries. The example script custom_query_example.py shows a practical implementation of AccountBalanceQuery as a custom query to illustrate the necessary abstract method implementations.

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)

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[Om7035]

@exploreriii can you check!!!

[exploreriii]

Hi @Om7035
We have a problem

We require all our examples to be runnable and 'work'

So, in this case, your a new sample class does not fully run - so the workflow fails

Either make sure that it runs, see

examples/query/account_balance_query.py
or
examples/query/account_balance_query_2.py
and
src/hiero_sdk_python/query/account_balance_query.py

or, add it to your query.md making it clear its a simplified example

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