Docs: Fix pretranslation guidelines link

[tahakotil]

Summary

• point the localizer guide to the localizer-facing pretranslation opt-in guidelines
• avoid redirecting readers to the administrator documentation

Verification

• confirmed the new URL returns HTTP 200

Fixes #4458

[flodolo]

Thanks for working on this and your first contribution to Pontoon 👍🏼

For the future, make sure to comment in the issue before starting the work, so other people can see that it's assigned.