chore: Mention developer docs in Contributing

[philipphofmann]

DESCRIBE YOUR PR

It took me quite a while to figure out how to run the developer docs locally. I would expect that info also in the Contributing.md.

Explain how to start the dev server for the developer docs in the Contributing.md and not just the environment.mdx.

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+

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

[vercel]

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

Name	Status	Preview	Comments	Updated (UTC)
develop-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 12, 2024 9:03am
sentry-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 12, 2024 9:03am

1 Skipped Deployment

Name	Status	Preview	Comments	Updated (UTC)
changelog	⬜️ Ignored (Inspect)			Dec 12, 2024 9:03am

[codecov]

Bundle Report

Changes will decrease total bundle size by 15 bytes (-0.0%) ⬇️. This is within the configured threshold ✅

Detailed changes

Bundle name	Size	Change
sentry-docs-server-cjs	10.34MB	9 bytes (-0.0%) ⬇️
sentry-docs-client-array-push	9.27MB	6 bytes (-0.0%) ⬇️