Add draft PR guidelines to CONTRIBUTING.md

[Copilot]

Recurring issues with draft PRs lacking clear descriptions or intent have caused confusion about when and how to comment. Contributors were using draft status as a shield from feedback rather than a tool for early collaboration.

Changes

Added "Using Draft Pull Requests" section to CONTRIBUTING.md clarifying:

• What draft status means: Work in progress, seeking early feedback, requesting help, sharing progress
• What it does NOT mean: Not a dump for unclear changes, not a shield from comments, not a substitute for local development
• Requirements for drafts: Clear intent/description, understandable changes, specific questions if help needed, reasonable completeness for meaningful review
• Conversion guidance: Reference to existing PR checklist when ready for final review

Positioning

Placed after "Creating a Pull Request" section for logical workflow progression. Links to existing "What does a valid PR look like?" checklist to clarify transition expectations from draft to ready-for-review.

Original prompt

We have an recurring problem with raft PRs and therefore like to clarify the contributing file.

Take this as examples:

• Migrate org.eclipse.ui.tests.navigator from JUnit 4 to JUnit 5 eclipse-platform/eclipse.platform.ui#3500
• [WIP] Proper selection coloring in JFace Structured Viewers eclipse-platform/eclipse.platform.ui#3659
• Sharp rendering of SVGs passed as InputStream eclipse-platform/eclipse.platform.swt#2935

These all where initally created as a draft with insufficent description or unclear state and people are confused and then argued that its a draft PR andone should probabbly not look at it.

For me a draft does not mean "leave me alone with your stupid comments" it is more something I like to share with the community, maybe getting some early feedback or help with some issues I'D like to solve.

A draft means it's not ready for submission (for whatever reason, e.g. not fully working, incomplete in tests), but of course others should be allowed to make comments and should be able to understand the intend and current state.It's not a dump for changes, one can use the local computer for that or simply just push changes to a fork's branch (and not create a PR from it).But every PR should be clear in it's intend.

Please help to clarify the contributing docs so we can use them as a reference fior further cases.

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