feat(relay): Add document describing span and transaction quotas

[Dav1dde]

DESCRIBE YOUR PR

Description of how span and transaction outcomes are (to be) handled within Relay.

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:
• [X] None: Not urgent, can wait up to 1 week+

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:

• [x] 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

[vercel[bot]]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
changelog	❌ Failed (Inspect)			Oct 29, 2024 2:50pm
develop-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Oct 29, 2024 2:50pm

1 Skipped Deployment

Name	Status	Preview	Comments	Updated (UTC)
sentry-docs	⬜️ Ignored (Inspect)	Visit Preview		Oct 29, 2024 2:50pm