docs: add LC0100 - Global variable could be local

[someC0d3r]

Adds the public documentation for LC0100, GlobalVariableCouldBeLocal.

Summary

The new rule page documents:

• why procedure-only global variables should be moved to local scope;
• before-and-after AL examples;
• the conditions under which LC0100 reports a diagnostic;
• conservative exclusions where retained object state may still matter;
• type-aware diagnostic messages, including quoted record type names;
• the separate wording used for immutable labels;
• that the rule is disabled by default and has no code action.

LC0100 is also added to the LinterCop rule index and linked to Discussion #499.

Verification

• The Hugo site build passed successfully with 164 pages.
• The message examples were checked against the analyzer output.
• The documented behavior was validated with the Microsoft Base Application and Microsoft Subcontracting App.

Companion analyzer draft PR: ALCops/Analyzers#506

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This documentation and draft PR were created with Codex using GPT-5.6 Sol with Ultra reasoning.

[someC0d3r]

reopened due to main PR ALCops/Analyzers#506 got also reopened