docs: Require issue discussion before PRs

[sentry-junior]

📜 Description

Asks external contributors to discuss a change with a maintainer in a GitHub issue before opening a PR. PRs without that prior discussion may be closed.

Changes to CONTRIBUTING.md:

• Replaces the "We love pull requests from everyone" intro with a short welcome and a Discord link (discord.com/invite/sentry).
• Adds a Submitting Changes section with:
  • a TL;DR: discuss in an issue first; PRs without prior discussion may be closed
  • Before You Start: find or create an issue, agree on the approach with a maintainer, check the assignee
  • Making Your Contribution: tests are required for non-trivial PRs, don't bump version numbers, open a draft PR that links the issue, follow the commit message format, keep PRs focused
  • AI Use, moved up from the end of the file. It now also says contributors must understand the changes they propose, not just review and test them.

💡 Motivation and Context

Mirrors getsentry/sentry-cocoa#9396 so the mobile/Java SDKs share the same contribution policy, with only the repo name changed. The wording says we may close PRs rather than promising automatic closing, because the validate-pr workflow only posts an advisory comment.

💚 How did you test it?

Docs-only change; reviewed the rendered Markdown structure.

#skip-changelog

via Chris Aigner.

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🔗 App Name	App ID	Version	Configuration
SDK Size	io.sentry.tests.size	8.61.0 (1)	release

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