Improve expansion internals documentation with high-level flow diagram and code references

[Copilot]

• Analyze existing docs/license-contributing/expansion-internals.md and understand what's missing
• Add mermaid diagram support to mkdocs.yml
• Update expansion-internals.md with:
  • High-level flow diagram (mermaid flowchart)
  • "Key components" table with source file references (wrapped in !!! Implementation admonition)
  • Code file references in Building Bicep, Visiting definitions, Ordering resources, Evaluating expressions sections (wrapped in !!! Implementation admonitions)
• Update docs/license-contributing/index.md to add link to expansion-internals
• Update docs/changelog.md with entry for issue Improve documentation for expansion internals #3715

Original prompt

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This section details on the original issue you should resolve

<issue_title>Improve documentation for expansion internals</issue_title>
<issue_description>Expand docs for contributions to map out a high level flow of. How expansion works and steps through processing Bicep code. </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Improve documentation for expansion internals #3715

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[BernieWhite]

@copilot For cases that are extended references to code or class implementations use the implementation admonition.

Do this ben creating a block such as:

!!! Implementation
    Further details and class references here.
    Second line.

[Copilot]

@copilot For cases that are extended references to code or class implementations use the implementation admonition.

Do this ben creating a block such as:
...

Done in c0032a2. All extended code/class reference blocks — the key components table, BicepHelper details, lazy-loading classes, ResourceDependencyGraph, and the expression evaluation classes list — have been wrapped in !!! Implementation admonitions.