docs: add upgrade guide for Imposter 5

[outofcoffee]

Summary

• New docs page docs/upgrade_v5.md covering the upgrade path from Imposter 4 to Imposter 5
• Explains the JVM-to-Go engine change, Groovy-to-JavaScript script migration, required config file changes, and the IMPOSTER_SUPPORT_LEGACY_CONFIG flag
• Adds the new page to the Operational topics section in mkdocs.yml

Test plan

• Build docs locally and verify the new page renders and appears in the nav
• Review the config before/after examples for accuracy