Adding image update strategy in SaaS runner docs

What does this MR do and why?

Updating the docs with our new image update strategy. gitlab-org/ci-cd/shared-runners/infrastructure#124 (closed)

Update 3 files

• /doc/ci/runners/saas/windows_saas_runner.md
• /doc/ci/runners/saas/macos_saas_runner.md
• /doc/ci/runners/index.md

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