feat(python): Start the 3.x migration guide

[sentrivana]

DESCRIBE YOUR PR

This is a WIP PR, I'll add reviewers to it as soon as it's ready (weeks/months).

We'll release a new major of the Python SDK in a couple of months. Preparing a migration guide to go with it.

IS YOUR CHANGE URGENT?

Help us prioritize incoming PRs by letting us know when the change needs to go live.

• [ ] Urgent deadline (GA date, etc.):
• [ ] Other deadline:
• [ ] None: Not urgent, can wait up to 1 week+
• [x] Not ready yet

SLA

• Teamwork makes the dream work, so please add a reviewer to your PRs.
• Please give the docs team up to 1 week to review your PR unless you've added an urgent due date to it. Thanks in advance for your help!

PRE-MERGE CHECKLIST

Make sure you've checked the following before merging your changes:

• [ ] Checked Vercel preview for correctness, including links
• [ ] PR was reviewed and approved by any necessary SMEs (subject matter experts)
• [ ] PR was reviewed and approved by a member of the Sentry docs team

LEGAL BOILERPLATE

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

EXTRA RESOURCES

• Sentry Docs contributor guide