new doc: add module declaration example for bzlmod / MODULE.bazel

[tomsoir]

This document contains a simple example code snippet of aspects_rules_webpack module declaration that should help with the migration from the old WORKSPACE.bazel to MODULE.bazel.

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Changes are visible to end-users: yes/no

• Searched for relevant documentation and updated as needed: yes/no
• Breaking change (forces users to change their own code or config): yes/no
• Suggested release notes appear below: yes/no

Test plan

• Covered by existing test cases
• New test cases added
• Manual testing; please provide instructions so we can reproduce:

[CLAassistant]

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